https://aquafishtips.blogspot.com/2016/03/marine-quarium-basics-biological-filtration.html

Filtration can be considered the process of keeping aquarium water fit for its inhabitants to live in and, as such, is one of the most important aspects of a marine system. Any aquatic organism living in a fixed volume of water will have effects on the chemistry of its environment, and these effects will typically be deleterious. Waste products will accumulate in the water as they are excreted from the organism, and beneficial components will be depleted as the organism uses them. If nothing is done, the end result will he water conditions that are drastically different from their starting point and harmful to aquarium inhabitants. Filtration addresses the accumulation of waste products in the aquarium water. In doing so, it overlaps with skimming, which is discussed in a future article.