Chemical
aquarium filtration is needed because a number of dissolved, invisible
compounds accumulate in aquarium water and they can't be removed by mechanical
filtration. These compounds are not toxic to the fish but can inhibit their
growth and cause chronic, low-level stress that eventually leads to disease.
Most of these compounds are dissolved organic substances produced by natural
biological decay.
The
dissolved organic substances eventually reach concentrations high enough to
become visible as a yellowish tinge in the water. You can see this when a sheet
of white paper is held behind the tank so that half of it is viewed through the
water. If the water is healthy for the fish, the paper viewed through the water
will be as white as the other half; if not, the paper will have a yellowish
cast to it.
Chemical
filtration removes many, but not all, of these compounds. However, some
substances that affect the growth of the fish can only be removed by making
partial water changes on a regular basis. If this isn't done, the fish will
never grow to normal adult size. This stunted growth will result in fish that
never achieve the beauty of mature fish, and it can cause other related health
problems.
