How Do Aquarium Bio Filters Work?

Biological Filtration

An aquarium bio filter is a filtration system that gives healthy bacteria a place to grow and multiply. This can be as simple as a live rock in a saltwater aquarium that gives bacteria crevices and holes in which to grow, to something a bit more complex like an expensive box filter on the back of the aquarium that has a plate on it to grow these bacteria. Because bacteria require oxygen to grow and thrive, a biological filtration system will also aerate the water, providing oxygen for your pets, plants and the bacteria.

Why do you need a bio filter? Your fish tank has a potentially deadly cycle that occurs every day. Through the natural digestive processes, your fish will excrete urine and other waste products, which contain high levels of ammonia. This ammonia is toxic to your fish, so you need a way to remove it. However, you cannot see it, so manually removing it is impossible. The only way to remove it is through biological filtration, which relies on the good bacteria that grow in a healthy fish tank. The first type of helpful bacteria your tank could have will consume the ammonia and turn it into nitrite. Nitrite is also harmful to your fish, although not quite as toxic as ammonia. In a healthy fish tank, another type of bacteria will grow that change nitrite into nitrate, which is harmless to your fish. However, without proper biological filtration, these healthy bacteria will not grow, and your fish could die from their own waste products.

Most aquarium filters serve as biological filters. In a saltwater tank, live rock is the most common type of biological filter. For freshwater tanks, undergravel filters provide the gravel bed as a place for bacteria to grow, pulling water through the gravel to expose it to the bacteria. Because undergravel filters are often sold with beginner aquarium kits, many assume they are not effective, but they work very well. A sponge filter is a filter that drives water through a cellulose sponge. The holes in the sponge give bacteria the perfect place to grow. These are ideal for small aquariums, like a breeding tank, where an undergravel system is not appropriate. The power filter, which is the hang-over-the-back waterfall type filter, is yet another option. These have a filter wool insert that gives bacteria a place to grow. Many also come with a foam wheel that provides another place for bacteria growth. Because the filter inserts have to be periodically changed, many aquarium experts, like those at Aquarium Pros, do not feel these provide adequate biological filtration. Finally, wet-dry filters are another biological filter option. In this filter, water enters several chambers housing biological media. As the water is exposed to the bacteria, the ammonia and nitrite are filtered out, and the clean water is sent back to the aquarium.