Biological filtration, mechanical filtration, chemical filtration (adsorption), and disinfection are four major processes used to maintain closed water systems (Spotte 1979). Biological filtration involves heterotrophic and autotrophic bacteria that convert nitrogenous organic compounds (fish excreta, consisting primarily of ammonia) into nitrites and further to the less toxic nitrates (Stoskopf 1993b). Investigators should monitor the nitrification process, which is affected by many factors, including temperature, pH, dissolved
oxygen, salinity, surface area of the filtrant, and bactericidal and parasiticidal agents.