In the rivers and streams in which many aquatic plants grow, the substrates vary depending on the environmental and geological conditions of the river system and the local area. What do you know about freshwater aquarium substrates?
Aquatic plants are often found in sandy, muddy, or gravel beds. An important point to bear in mind is that in virtually all natural areas the substrate is usually warmer than the surrounding environment. This happens because the heat from the sun is absorbed and retained by the substrate. The difference may be less than one degree, but it is enough to create convection currents between the substrate and the water. These currents slowly and continually move water down through the substrate (where it warms up slightly) and back up into the main water body as it cools. As water passes through the substrate it takes nutrients with it, giving the roots access to a continual supply of nutrients.
In many streams and rivers, there are certain areas where plants grow together in groups while just a short distance away there are no plants at all. This happens because natural springs occur along the river or stream system and water from other areas enters through the ground. Most of these springs are hidden from view and may carry only a trickle of water, but the water is rich in organic and mineral nutrients, which the plants quickly absorb through their roots.
The dense muddy substrate found in many places provides an ideal anchoring medium that holds plants firmly in position. In natural conditions, the roots may grow far wider and deeper than they possibly could in the aquarium. When keeping some larger plants, such as many of the larger Echinodorus species, take into account the fact that their roots will quickly spread and literally "take over" the aquarium substrate if allowed to do so.
So which substrates are best? This is not an easy question to answer. Some plants do not need any specialized substrates while a few need no substrate at all! However, for the most part, a mixture of substrates will create an environment suitable for all the plants in the aquarium. The points to consider are the size and shape of the particles, the depth of the substrate layer, and its mineral and organic content.
If the substrate is too fine it may compact, halting the movement of oxygen and nutrients, and causing damage to the root structure.
A suitable aquarium substrate should have a particle size of about 0.04-0.12 in (1-3 mm) and be rounded in shape; sharp substrate particles can damage roots. The only exception is sand, which can be used as a thin bottom layer to support heating cables.
Generally speaking, foreground plants do not produce long roots, so it is possible to slope the substrate upward toward the back of the aquarium. This also makes the aquarium appear deeper than it is. A good substrate depth Is 2.4-4 In (6-10 cm).
More importantly, a substrate should not contain harmful minerals, most notably, compounds with a high calcium content. Limestone and coral-based substrates, often available for marine aquariums, are high in calcium and should never be used In a freshwater planted aquarium. Substrates such as these will Increase the alkalinity and pH of the water, making it harder for plants to obtain nutrients and C02.
Base substrate
A base substrate is necessary only when a heating cable is present. To be effective, the heat from the cable must be well distributed through the surrounding substrate and this Is easy to achieve using a very fine substrate with a particle size of about 0.04 in (1 mm). Sand Is Ideal for this purpose but use just enough to cover the heating cable.
Main rooting substrate
The main body of the substrate is used principally as a rooting medium, but also for the uptake of nutrients. This substrate should be compact enough to prevent excess water movement and oxygenation, but loose enough so that it does not become stagnant and produce toxins.
A grade of 0.08-0.12 in (2-3 mm) Is suitable and the substrate can be mixed with nutrient-rich additives. More than one main substrate can be used.
Nutrient-rich substrate
This substrate should provide the plants with a constant and long-lasting supply of a range of nutrients. Depending on the concentration of nutrients present, use either a wafer-thin layer or a layer 0.4-1.6 In (1-4 cm) deep. Nutrient-rich substrates are usually highly compact and soil-like, so "sandwich" them between other substrates to prevent muddying of the water. Some nutrient-rich substrates can be mixed with the main rooting substrate.
Top level substrate
This will be the most visible substrate and need only be a thin layer placed on top of other substrates. It need not provide any function for the plants and may be used purely for aesthetic purposes.
Pea gravel
The most common form of aquarium gravel is called pea gravel, due to its smooth, rounded appearance. Pea gravel Is available in a number of different grades, although only the smaller grades should be used as the main substrate. Although pea gravel Is generally inert, It does contain some rock types that may affect the hardness of the water. Aquatic plants will not benefit much from pea gravel, except as a rooting substrate.
Quartz gravel
Quartz is a completely inert substrate and ideal as a main rooting medium and/or top layer for the planted aquarium. Quartz gravel is often sold and labeled as lime-free substrate and usually available In grades of 0.04-0.12 In (1-3 mm). In smaller aquariums, or tanks with few plants or simple designs, quartz gravel Is the best option for a sole substrate. It makes a better alternative to pea gravel due to Its Inert nature and smaller grade size, which provides a better support for plant roots.
Sand
Fine-grade sand can cause problems If used as the sole substrate In an aquarium. Over time it will compact, preventing water movement and causing anaerobic conditions, which result In stagnation and the release of toxins. Stirring the sand gently and regularly will prevent this problem although most plants do not appreciate constant disturbance. However, due to its small size, sand is very effective at distributing heat In a localized area, such as around a heating cable. If the sand layer is not too deep (about 1.6 ln/4 cm), convection currents from a heating cable will ensure a small amount of water movement through the substrate, allowing useful anaerobic conditions to develop without stagnation. When using sand, be sure to choose a completely inert form. Many commercial sands contain traces of lime or calcareous materials, although most of the products sold by aquatic retailers are safe. Silver sand Is commonly available and suitable for the aquarium.
Laterite/day substrates Clay-based substrates, often called latente, are usually available as a substrate additive, reflecting the fact that only a small amount is needed. These substrate additives are usually very fine, sometimes even powdery, and reddish colored. They release a number of nutrients, Including Iron, over a long period. Clay-based substrates are best used as a layer toward the bottom third of the substrate or mixed in with the lower half of the main substrate. This Is where the fine roots absorb the nutrients.
Nutrient-rich substrates
Certain planting substrates are specifically designed for aquarium plants. Most of these substrates are laterite-based but contain additional organic and mineral materials that release a number of nutrients over long periods of time. Special planting substrates are available as additives and as main substrates.
Use additives In small quantities, either as a thin layer or mixed with the main substrate.
Soil-based substrate
Generally speaking, beginners should avoid using soil in the aquarium, as results can vary wildly. However, more experienced aquarists find that soil can be one of the best long-term planting substrates. It contains large amounts of carbon and iron, both readily used by aquatic plants, as well as a number of other nutrients that are slowly released or retained by the soil. If you do decide to use soil, a 1-1.5 In (2.5-3.75 cm) layer of soil used as a base layer and main substrate, topped with 1 in (2.5 cm) of fine gravel, will suit most aquariums. The safest strategy is to use only sterilized potting mixture - not garden soil - to prevent contamination.
Due to the breakdown of organic matter within the soil, low levels of C02 are constantly released. In many planted aquariums with a soil substrate, additional C02 fertilization is not needed and neither are additional substrates or iron fertilization.
During the first few weeks of soil use, the aquarium may experience a high release of nutrients and organic matter.
Although plants should be able to cope with this, it can occasionally cause problems or produce dangerous conditions for fish. For this reason, it is a good idea to wait a few weeks before introducing any fish, and to filter the water using carbon or carry out regular water changes.
Although potting mixture can become an excellent planting medium, and using even small amounts of nutrient-rich substrates may be more expensive, it could be argued that specifically designed substrates are a safer and more predictable option than soil substrates.
Other substrates
In addition to these common substrates, there are some other, less-often used substrates available for aquariums and aquarium plants. Many "brand-name" substrates are variations or combinations of nutrient-rich and inert substrates. Ready-made mixes are fine for aquarium plants and offer an easier alternative for beginner aquarists.
Peat is often promoted as a partial substrate for aquatic plants. Much like a combination of soil and nutrient-rich substrates, peat is very high in organic matter and provides plants with a wealth of nutrients. However, it tends to release some nutrients quickly, which can encourage algae, so it may not be suitable for long-term use.
Large-grade and sharp substrates are not particularly suitable, nor are colored substrates as they have a smooth surface that hinders the growth of useful bacteria.
A heating cable does not need a thermostat and can be left on continuously. The heat output and power consumption are very low, so the overall aquarium temperature should not be significantly affected. Substrate heating is not a vital part of a good planting substrate, but where thin layers of nutrient-rich substrates are used, the currents produced by a heating cable will significantly improve the distribution of the nutrients.
Over time, organic waste and debris will collect in the substrate, making it denser and creating anaerobic conditions. In a well-planted aquarium, the majority of organic debris will be broken down by bacteria and the resulting nutrients are taken up by plant roots, which in return release small amounts of oxygen into the substrate and help to prevent stagnation. So the substrate in a well-planted aquarium may actually last longer than a substrate without the benefit of dense planting. However, a buildup of organic debris in the uppermost layer may not be broken down very quickly and plant roots are not present In sufficient quantities in this area to keep the substrate oxygenated. Regular gentle stirring or siphoning of the top layer will help to remove debris and keep the substrate clean and healthy. Only the top layer should be regularly disturbed; the deeper substrate contains the dense plant roots that will not appreciate the disruption.
If the main substrate, or substrates, are very fine - about 0.08 In (2 mm) -they will compact over time, reducing the water flow. This, in turn, allows toxic gases (produced by the breakdown of organics) to build up. These gases will damage plant roots so that they are unable to release oxygen and stagnant areas of the substrate are created. But a certain amount of anaerobic substrate is not necessarily a bad thing (see the panel opposite). Where the fine-grade substrate is used, check the density every few months by pushing a finger down through the substrate. This should be possible with only a little resistance. If it becomes difficult, gently disturb and loosen the deeper substrate.
The nutrient-containing elements of a substrate will diminish over time and will need to be replaced. Most nutrient-rich substrates will last for about three years before they begin to run out and become ineffective. However, this does vary depending on a number of factors, such as the quantity and type of plants, additional fertilization, the quantity and waste from fish, water conditions, etc. When the substrate needs replacing, the plants will need to be removed and replanted. This process will be damaging for some of the plants and you should take great care, closely following the guidelines for replanting.
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| Aquarium substrates for plants |
Aquatic plants are often found in sandy, muddy, or gravel beds. An important point to bear in mind is that in virtually all natural areas the substrate is usually warmer than the surrounding environment. This happens because the heat from the sun is absorbed and retained by the substrate. The difference may be less than one degree, but it is enough to create convection currents between the substrate and the water. These currents slowly and continually move water down through the substrate (where it warms up slightly) and back up into the main water body as it cools. As water passes through the substrate it takes nutrients with it, giving the roots access to a continual supply of nutrients.
In many streams and rivers, there are certain areas where plants grow together in groups while just a short distance away there are no plants at all. This happens because natural springs occur along the river or stream system and water from other areas enters through the ground. Most of these springs are hidden from view and may carry only a trickle of water, but the water is rich in organic and mineral nutrients, which the plants quickly absorb through their roots.
The dense muddy substrate found in many places provides an ideal anchoring medium that holds plants firmly in position. In natural conditions, the roots may grow far wider and deeper than they possibly could in the aquarium. When keeping some larger plants, such as many of the larger Echinodorus species, take into account the fact that their roots will quickly spread and literally "take over" the aquarium substrate if allowed to do so.
Substrates in the aquarium
In an average aquarium, the substrate is likely to be a fairly straightforward affair, usually a simple covering of pea gravel. Plants use the substrate not only as a place to root, but also as a source of nutrients and, in some cases, a medium through which to reproduce. The root systems of aquatic plants vary between species, but all are highly evolved to work effectively in a given natural environment. Most of these environments are nothing like those found in an aquarium with a simple pea gravel substrate. Difficulties encountered when keeping aquatic plants can often be attributed to the lack of a good, useful substrate. Clean, inert gravel creates a fairly biologically inactive substrate. Because the water flows easily through such a medium, it removes nutrients, cools the plant roots, and creates an oxygen-rich area, all of which are undesirable and hinder the development of aquatic plant roots.So which substrates are best? This is not an easy question to answer. Some plants do not need any specialized substrates while a few need no substrate at all! However, for the most part, a mixture of substrates will create an environment suitable for all the plants in the aquarium. The points to consider are the size and shape of the particles, the depth of the substrate layer, and its mineral and organic content.
Size and shape
If the particle size of the substrate is wrong, it may cause problems for aquatic plants. A substrate made up of particles that are too large will allow water to pass through easily, removing nutrients. Furthermore, debris will collect in the gaps between the particles, which may muddy the water. Large-grade substrates also cause problems for the growth of long roots and should be used only as a thin top layer.If the substrate is too fine it may compact, halting the movement of oxygen and nutrients, and causing damage to the root structure.
A suitable aquarium substrate should have a particle size of about 0.04-0.12 in (1-3 mm) and be rounded in shape; sharp substrate particles can damage roots. The only exception is sand, which can be used as a thin bottom layer to support heating cables.
Substrate depth
Substrate depth does vary a little, depending on which species of plant you are keeping. Plants that produce long roots, such as Echinodorus species and some cryptocoryne, will need a substrate deep enough for the roots to penetrate. If the substrate Is too shallow, the roots of these plants will become dense and tangled. In this situation, the plant cannot obtain nutrients and the roots will become starved of oxygen.Generally speaking, foreground plants do not produce long roots, so it is possible to slope the substrate upward toward the back of the aquarium. This also makes the aquarium appear deeper than it is. A good substrate depth Is 2.4-4 In (6-10 cm).
Mineral content
Plants require minerals In small amounts, but it is difficult to provide these through the substrate, although some nutrient-rich substrates do contain the essential minerals that aquatic plants require. In general, the quantities of minerals required by plants are usually readily available In tap water. However, If the source water for your aquarium Is relatively soft it may be lacking in these minerals, in which case you can use liquid fertilizers.More importantly, a substrate should not contain harmful minerals, most notably, compounds with a high calcium content. Limestone and coral-based substrates, often available for marine aquariums, are high in calcium and should never be used In a freshwater planted aquarium. Substrates such as these will Increase the alkalinity and pH of the water, making it harder for plants to obtain nutrients and C02.
Organic content
The organic content of a substrate includes organic nutrients, as well as waste matter from the aquarium (mainly from fish). A substrate without any organic matter Is simply an "anchor" for the plants and of little other use. You can add organic matter by using a nutrient-rich substrate, which can either be mixed with the main substrate or arranged as a layer between two substrates. Soil and peat have a very high organic content, so use them with care to avoid overloading the aquarium with organic matter.Choosing substrates
Several different substrates are available for the aquarium, and making the right choices can be a little tricky. It is possible to have a reasonably good planting substrate using just one medium, although combining a number of substrates will usually produce much better results. When mixing substrates, bear in mind that each one should have a place and a function. Assessing substrates on the basis of their usefulness for a specific purpose will make It easier to choose the right ones.| SUBSTRATES AND THEIR USES | ||||
| Type of substrate | Base substrate | Main rooting substrate | Top substrate | Nutrient content |
| Pea gravel | Not suitable | Small-grade pea gravel can be used as the main substrate. | Ideal top-level substrate. | None |
| Quartz gravel | Small grades can be used with a heating cable. | Ideal as a main substrate. | Looks good as a top layer, but due to the small grade, mulm will collect more readily. Not normally a major problem. | None |
| Sand | Ideal for use with a heating cable. | Not suitable due to compaction. | Not suitable | None |
| Laterite/clay substrates | Not suitable | Not suitable | Not suitable | High |
| Nutrient-rich substrates | Not suitable | Some nutrient-rich substrates are designed to be used in large quantities as the main substrate. If the grade size is greater than 0.16 in (4 mm), mix it with a smaller lime-free substrate. | Not suitable | High |
| Soil-based substrates | Suitable | Suitable if covered by a 1 in (2.5 cm) layer of gravel. | Not suitable | High, may be lacking hardwater nutrients. |
Base substrate
A base substrate is necessary only when a heating cable is present. To be effective, the heat from the cable must be well distributed through the surrounding substrate and this Is easy to achieve using a very fine substrate with a particle size of about 0.04 in (1 mm). Sand Is Ideal for this purpose but use just enough to cover the heating cable.
Main rooting substrate
The main body of the substrate is used principally as a rooting medium, but also for the uptake of nutrients. This substrate should be compact enough to prevent excess water movement and oxygenation, but loose enough so that it does not become stagnant and produce toxins.
A grade of 0.08-0.12 in (2-3 mm) Is suitable and the substrate can be mixed with nutrient-rich additives. More than one main substrate can be used.
Nutrient-rich substrate
This substrate should provide the plants with a constant and long-lasting supply of a range of nutrients. Depending on the concentration of nutrients present, use either a wafer-thin layer or a layer 0.4-1.6 In (1-4 cm) deep. Nutrient-rich substrates are usually highly compact and soil-like, so "sandwich" them between other substrates to prevent muddying of the water. Some nutrient-rich substrates can be mixed with the main rooting substrate.
Top level substrate
This will be the most visible substrate and need only be a thin layer placed on top of other substrates. It need not provide any function for the plants and may be used purely for aesthetic purposes.
Types of substrate
Here we look at commonly available aquarium substrates and their suitability.Pea gravel
The most common form of aquarium gravel is called pea gravel, due to its smooth, rounded appearance. Pea gravel Is available in a number of different grades, although only the smaller grades should be used as the main substrate. Although pea gravel Is generally inert, It does contain some rock types that may affect the hardness of the water. Aquatic plants will not benefit much from pea gravel, except as a rooting substrate.
Quartz gravel
Quartz is a completely inert substrate and ideal as a main rooting medium and/or top layer for the planted aquarium. Quartz gravel is often sold and labeled as lime-free substrate and usually available In grades of 0.04-0.12 In (1-3 mm). In smaller aquariums, or tanks with few plants or simple designs, quartz gravel Is the best option for a sole substrate. It makes a better alternative to pea gravel due to Its Inert nature and smaller grade size, which provides a better support for plant roots.
Sand
Fine-grade sand can cause problems If used as the sole substrate In an aquarium. Over time it will compact, preventing water movement and causing anaerobic conditions, which result In stagnation and the release of toxins. Stirring the sand gently and regularly will prevent this problem although most plants do not appreciate constant disturbance. However, due to its small size, sand is very effective at distributing heat In a localized area, such as around a heating cable. If the sand layer is not too deep (about 1.6 ln/4 cm), convection currents from a heating cable will ensure a small amount of water movement through the substrate, allowing useful anaerobic conditions to develop without stagnation. When using sand, be sure to choose a completely inert form. Many commercial sands contain traces of lime or calcareous materials, although most of the products sold by aquatic retailers are safe. Silver sand Is commonly available and suitable for the aquarium.
Laterite/day substrates Clay-based substrates, often called latente, are usually available as a substrate additive, reflecting the fact that only a small amount is needed. These substrate additives are usually very fine, sometimes even powdery, and reddish colored. They release a number of nutrients, Including Iron, over a long period. Clay-based substrates are best used as a layer toward the bottom third of the substrate or mixed in with the lower half of the main substrate. This Is where the fine roots absorb the nutrients.
Nutrient-rich substrates
Certain planting substrates are specifically designed for aquarium plants. Most of these substrates are laterite-based but contain additional organic and mineral materials that release a number of nutrients over long periods of time. Special planting substrates are available as additives and as main substrates.
Use additives In small quantities, either as a thin layer or mixed with the main substrate.
Soil-based substrate
Generally speaking, beginners should avoid using soil in the aquarium, as results can vary wildly. However, more experienced aquarists find that soil can be one of the best long-term planting substrates. It contains large amounts of carbon and iron, both readily used by aquatic plants, as well as a number of other nutrients that are slowly released or retained by the soil. If you do decide to use soil, a 1-1.5 In (2.5-3.75 cm) layer of soil used as a base layer and main substrate, topped with 1 in (2.5 cm) of fine gravel, will suit most aquariums. The safest strategy is to use only sterilized potting mixture - not garden soil - to prevent contamination.
Due to the breakdown of organic matter within the soil, low levels of C02 are constantly released. In many planted aquariums with a soil substrate, additional C02 fertilization is not needed and neither are additional substrates or iron fertilization.
During the first few weeks of soil use, the aquarium may experience a high release of nutrients and organic matter.
Although plants should be able to cope with this, it can occasionally cause problems or produce dangerous conditions for fish. For this reason, it is a good idea to wait a few weeks before introducing any fish, and to filter the water using carbon or carry out regular water changes.
Although potting mixture can become an excellent planting medium, and using even small amounts of nutrient-rich substrates may be more expensive, it could be argued that specifically designed substrates are a safer and more predictable option than soil substrates.
Other substrates
In addition to these common substrates, there are some other, less-often used substrates available for aquariums and aquarium plants. Many "brand-name" substrates are variations or combinations of nutrient-rich and inert substrates. Ready-made mixes are fine for aquarium plants and offer an easier alternative for beginner aquarists.
Peat is often promoted as a partial substrate for aquatic plants. Much like a combination of soil and nutrient-rich substrates, peat is very high in organic matter and provides plants with a wealth of nutrients. However, it tends to release some nutrients quickly, which can encourage algae, so it may not be suitable for long-term use.
Large-grade and sharp substrates are not particularly suitable, nor are colored substrates as they have a smooth surface that hinders the growth of useful bacteria.
Substrate heating
As we have seen, the substrate is slightly warmer than the main body of water in natural rivers and streams. The currents produced as a result of the temperature differences help to move nutrients around the roots of the plants. Similar currents can be created in the aquarium using a substrate heating cable. Placed at the base of the substrate, it produces a very gentle heat that raises the temperature of the surrounding substrate. This heat rises through the water in the substrate to the surface and cooler water is drawn down through the substrate so the circulation begins again. The substrate around the heating cable should be fairly dense so that the heat is distributed quickly and the cable is supported and held firmly in place. A very fine substrate such as sand is ideal for this purpose.A heating cable does not need a thermostat and can be left on continuously. The heat output and power consumption are very low, so the overall aquarium temperature should not be significantly affected. Substrate heating is not a vital part of a good planting substrate, but where thin layers of nutrient-rich substrates are used, the currents produced by a heating cable will significantly improve the distribution of the nutrients.
Maintaining the substrate
A good substrate will require little maintenance and is often best left undisturbed for the majority of the aquarium's life. Once an aquarium is well established and if the fish and plants are healthy, it is a major disruption to alter the substrate. It is, therefore, important to choose the correct substrate right from the start.Over time, organic waste and debris will collect in the substrate, making it denser and creating anaerobic conditions. In a well-planted aquarium, the majority of organic debris will be broken down by bacteria and the resulting nutrients are taken up by plant roots, which in return release small amounts of oxygen into the substrate and help to prevent stagnation. So the substrate in a well-planted aquarium may actually last longer than a substrate without the benefit of dense planting. However, a buildup of organic debris in the uppermost layer may not be broken down very quickly and plant roots are not present In sufficient quantities in this area to keep the substrate oxygenated. Regular gentle stirring or siphoning of the top layer will help to remove debris and keep the substrate clean and healthy. Only the top layer should be regularly disturbed; the deeper substrate contains the dense plant roots that will not appreciate the disruption.
If the main substrate, or substrates, are very fine - about 0.08 In (2 mm) -they will compact over time, reducing the water flow. This, in turn, allows toxic gases (produced by the breakdown of organics) to build up. These gases will damage plant roots so that they are unable to release oxygen and stagnant areas of the substrate are created. But a certain amount of anaerobic substrate is not necessarily a bad thing (see the panel opposite). Where the fine-grade substrate is used, check the density every few months by pushing a finger down through the substrate. This should be possible with only a little resistance. If it becomes difficult, gently disturb and loosen the deeper substrate.
The nutrient-containing elements of a substrate will diminish over time and will need to be replaced. Most nutrient-rich substrates will last for about three years before they begin to run out and become ineffective. However, this does vary depending on a number of factors, such as the quantity and type of plants, additional fertilization, the quantity and waste from fish, water conditions, etc. When the substrate needs replacing, the plants will need to be removed and replanted. This process will be damaging for some of the plants and you should take great care, closely following the guidelines for replanting.
ANAEROBIC CONDITIONS - GOOD OR BAD?
A substrate rich in organic material (waste matter and nutrient-rich substrates) will naturally contain large numbers of bacteria that break down these organics into usable nutrients. The majority of these bacteria quickly use up oxygen, with the result that the substrate becomes anaerobic. In anaerobic conditions, different types of bacteria form, which do not need to use large quantities of oxygen or can create their own oxygen. These anaerobic bacteria can release toxic gases, most notably hydrogen sulphide, which can cause plant roots to rot, damage fish health, and encourage algae to flourish.
However, anaerobic conditions also allow nutrients to become more readily available to plants by preventing the binding of nutrients with oxygen molecules. As the bacteria use up the nitrates, nitrogen is released, which is also an important plant nutrient.
A mixture of aerobic and anaerobic substrate zones can provide the benefits of both conditions. As long as the substrate is not too fine and compact and/or substrate heating is employed, the combination of a slow-moving current and the release of oxygen by plant roots should prevent the majority of the substrate from becoming anaerobic. Anaerobic patches will then appear in denser areas of the substrate without plant roots. Because these patches are small, they will not produce large amounts of toxic gases yet still allow nutrients to be produced and available to the plants. So a low-oxygen substrate is often best, where anaerobic conditions are allowed to develop in some places but not in others.
A substrate rich in organic material (waste matter and nutrient-rich substrates) will naturally contain large numbers of bacteria that break down these organics into usable nutrients. The majority of these bacteria quickly use up oxygen, with the result that the substrate becomes anaerobic. In anaerobic conditions, different types of bacteria form, which do not need to use large quantities of oxygen or can create their own oxygen. These anaerobic bacteria can release toxic gases, most notably hydrogen sulphide, which can cause plant roots to rot, damage fish health, and encourage algae to flourish.
However, anaerobic conditions also allow nutrients to become more readily available to plants by preventing the binding of nutrients with oxygen molecules. As the bacteria use up the nitrates, nitrogen is released, which is also an important plant nutrient.
A mixture of aerobic and anaerobic substrate zones can provide the benefits of both conditions. As long as the substrate is not too fine and compact and/or substrate heating is employed, the combination of a slow-moving current and the release of oxygen by plant roots should prevent the majority of the substrate from becoming anaerobic. Anaerobic patches will then appear in denser areas of the substrate without plant roots. Because these patches are small, they will not produce large amounts of toxic gases yet still allow nutrients to be produced and available to the plants. So a low-oxygen substrate is often best, where anaerobic conditions are allowed to develop in some places but not in others.