Aquarium biological filters are the most important filters to include in your aquarium setup. In fact, if you use only 1 type of filtration it should be biological. If you want your tank to be pristine and have tip top water quality for health fish then you will also want mechanical and chemical filtration.
Aquarium biological filters come in different forms and sizes but they all work by providing a surface for bacteria to grow on. The water is then pushed through the bacteria bed. The bacteria work to break down or filter the toxins in the water. That is why it is important to cycle your tank – this is what causes the bacteria to start and to add fish gradually in order to allow the bacteria to multiply at a rate that can keep up with the added waste produced by each fish.
The most inexpensive type of aquarium biological filters is a corner or box filter. These are those triangular shaped plastic boxes that sit inside the aquaruim in the corner filled with white floss. They are quite ugly but don’t cost much and work well. If you are spending a lot of money and time on your fish and aquarium decorations you’ll probably want to buy one of the other kinds which work just as well but look a lot better.
A sponge filter is another one of the more inexpensive aquarium biological filters. This type of filter usually sits outside the tank. The bacteria bed grows on the sponge and a pump is used to pull water through the sponge. Good for smaller tanks or tanks with a minimal amount of fish or as a second filter.
The undergravel filter is also a biological filter and the kind that I used in my salt water tank along with a power filter for mechanical and chemical filtration. With undergravel filtration, a bacteria bed is established in the gravel. The filter is actually a plastic slotted tray that sits on the bottom with the gravel on top of it (it needs to be installed when you first set up the tank). This creates an air space underneath the gravel. Tubes run up the sides from the tray and a pump is used to pull the water through the gravel. They work pretty good, but also pull all the waste down into the gravel so a vacuum must be used once a week to clean out the gravel.
Other types of aquarium biological filters include trickle filters that sit under the aquarium and work by having water gravity fed through the filter, canisters filters that also sit underneath the tank and can be filled with any number of materials to enhance your chemical or biological filtration and bio wheels. Bio wheels are a combination, chemical, biological and mechanical filtration in one unit that hangs off the back of the tank.