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.