What is an 'under gravel filter' for an aquarium?

What is an 'under gravel filter' for an aquarium?

Marc Grover (Professional Aquarist) gives expert video advice on: Generally, how do filters work in an aquarium?; What are the best filters for a small freshwater aquarium?; What are the best filters for a large aquarium? and more...

An under-gravel filter is pretty much exactly what the name implies. It's a filter that goes underneath the gravel in the aquarium. It's not a filter like you're thinking of, that water comes in and water gets pumped back out. It's a plate that sits underneath the gravel that houses nitrifying microscopic bacteria. Those bacteria convert toxic wastes, ammonia and nitrite, into nitrate which is not toxic or has a low toxicity to it. It's a biological filter. Biological filtration is a heart of every system. It is different than a mechanical filter, that is a pump that's pulling water out and cleaning it, then pushing it back in. An under-gravel filter is best used, or is most commonly used, in freshwater application and not with live plants because live plants will shoot their roots down and sometimes that'll kind of compromise the function of the under-gravel. It doesn't mean you can't use it for other systems, i.e. saltwater, it's just not super efficient and you can get away with not super efficient in freshwater. It's also one of the least expensive, biological filters you can get compared to the ones with saltwater; so that's why the trade off is less efficient, less cost, and that works better in freshwater.