How Do Aquarium Bio Filters Work?

A successful bio filter is much more than just a machine.
i inside a fish tank image by Yali Shi from

Aquarium bio filters work by letting organisms break down wastes, just like in nature. A biological filtration system provides several places for microorganisms to grow in your tank and can include plants and small invertebrate animals.

Filter Media

The most obvious part of your bio filter is the filter machine itself. There are a huge variety of types, but all contain a filtration medium made of carbon and a bio filtration medium. The bio filtration medium is a substrate for microorganisms to grow on. It usually looks like blue floss but might also be a sponge or rubbery pad. It's important to never use anything stronger than lukewarm water to clean this because you don't want to kill the little critters that'll grow in it and make it work.


Biofiltration does not stop with the filter. In fact, your mechanical filter is only the beginning. The most hard-working portion of your bio filter system is actually the gravel in your fish tank. This is where the bulk of the waste-munching microorganisms live. Gravel care is essential to the longevity of your aquarium and its inhabitants.

To build up a good bio filtration bed, you'll cure your gravel by letting your tank run with plants for about two weeks before adding any fish, then adding just 2 or 3 fish at a time with a couple weeks between each batch. This allows those microorganisms to establish themselves and get to work without overloading your new tank with waste.

Clean your gravel during water changes with a special siphon called a gravel vacuum and avoid using hot water or cleaning products in the tank.


Live plants make awesome additions to your aquarium bio filter system, so much so that some aquarium experts recommend reserving an algae wall in your tank, where you allow the green gunk to grow unmolested. A more appealing and decorative choice is to have a planted aquarium. There are a wide variety of potted, rooted and free-floating fish tank plants. The ones you choose will be based on the type of fish you want to keep, their behavior and habits. A special plant light is usually unnecessary.

Live Rock, Live Sand and Invertebrates

If you have a saltwater aquarium, you'll want to add live rock or live sand. The "live" refers to the tiny invertebrate animals -- some microscopic -- that live in this substrate. These keep your water quality high and your fish healthy by busily munching waste.

For a freshwater tank, you might consider adding snails, or fresh- or brackish water crustaceans like ghost shrimp or redclaw crabs (some snails and other minuscule invertebrates like hydras are guaranteed to hitch a ride in on your live plants).

If you are interested in welcoming invertebrate pets into your aquarium, you'll have to read up on them specifically -- they may require special diets and care, and some fish consider them a very tasty treat.

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