- Fish and marine invertebrates, such as crabs and living coral, pollute aquarium water with their metabolic waste. Aquarists should not overstock their aquariums, as these metabolic products are toxic to the animals which produce them. Ammonia is toxic even in trace quantities, while nitrite (NO2-) is only slightly less so. Marine aquariums should be stocked with fewer fish than in fresh water systems. Consider the "1-inch of fish per gallon" rule as an approximation only, as heavy bodied fish, which are messy feeders, place more strain on aquarium water than thinner bodied species do.
- Uneaten food pollutes aquarium water very quickly. Feed fish small amounts of food on a regular basis and only feed an amount that they can easily eat within a few minutes. Remove pieces of blanched spinach or similar leaves from the aquarium on a daily basis. Replace these with fresh leaves for herbivorous fish, but do not allow them to decompose. Remove large pieces of fresh sea food items, like prawn or calamari, after an hour if they are not consumed.
- Filter material should be rinsed or replaced on a regular basis. Mechanical filter material, such as filter floss, requires rinsing on a weekly basis and must be discarded and replaced if it is too dirty. Replace chemical filter material, like activated carbon, on a monthly basis. This prevents the toxins, which the carbon has absorbed into its pores, from leaching back into the aquarium. Filter material physically traps or absorbs harmful substances, and unless these materials are cleaned or replaced, the unwanted materials they contain remain in the water.
- Pollution in an aquarium can be diluted by regular water changes. The needs of each aquarium are different, because each is managed in a unique way. Generally speaking though, a weekly water change of 10 percent benefits most aquariums. Vacuum the aquarium gravel at this time, to remove dirt and detritus, which becomes easily trapped between the individual grains of sand. Fresh water that is added should ideally be the same temperature as the aquarium water.