Biological
aquarium filtration is the most important of all. The lack of effective
biological filtration is probably responsible for the deaths of more fish than
any other cause. The particular dissolved compounds controlled by biological
filtration are very toxic to fish even at low concentrations.
In
newly set up tanks, the effects of these compounds can kill fish very quickly.
In aquariums that have been running longer but are overstocked with fish, there
can be constant low levels of these compounds in the water. This creates
chronic, long-term physical stress, resulting in diseased and dying fish.
To
understand biological filtration, it is necessary to understand a basic process
in the aquarium: the nitrogen cycle. Ammonia is one of the key elements in the
nitrogen cycle. Fish produce ammonia directly both as a by-product of
respiration and as a waste product from the digestion of foods.
Solid
wastes are also converted into ammonia, which is why it is important to remove
them with mechanical filtration. Uneaten food, plant materials, and other
organic items that decay in the tank are also converted to ammonia. Ammonia, a
nitrogen-based compound, is extremely toxic. In an aquarium, it can build up
quickly and threaten all the fish in the tank.
