The theory is that aerobic bacteria covering the surface of the porous live rock and sand convert harmful ammonia (from fish and invertebrate waste) into nitrites, then nitrates, which are much less harmful to the tank's inhabitants.
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Through the process of diffusion, the nitrates move deep within the rock where they are converted by anaerobic bacteria to free nitrogen gas.
Although bona fide infectious diseases such as Ichthyophthirius multifiliis (Ick) do not spontaneously appear due to overcrowding itself, a disease-state results from elevated nitrates.
Live sand, a term used in aquarism, is natural reef coral sand populated with millions of beneficial bacteria and organisms which aid in the dissolving of organic wastes like ammonia, nitrites and nitrates produced by larger organisms in saltwater aquariums.
However, the purification system did not have bacteriostatic or bactericidal properties and it was incapable of removing impurities such as lithium, alcohols, ammonia, strong acids and bases or inorganic substances like sodium, lead, iron, arsenic and nitrates.