The above diagram is a graphic portrayal of the biological filter (nitrification) cycle. The actual toxicity levels and the number of days required for the completion of the cycle depends upon the aquarium size, the species of animals, size, number of fish therein, the amount of dissolved oxygen in the water, and the quality/efficiency of the biological filtration. The cycle does not begin until fish are added into the aquarium or pond!
The nitrification cycle is the process that transforms new nonfunctioning biological filter beds into biologically established filters.
• Initially the aquarium should be lightly populated with relatively inexpensive, hardy fish. We do not recommend feeder goldfish for this purpose as they are unlikely to survive the cycle. Being particularly sensitive to ammonia, scavengers, algae eaters and scaleless species should not be included among the first inhabitants.
• Fish digestion, respiration and decomposition of uneaten food begins to produce ammonia (NH3) Once ammonia becomes detectable, no other fish should be added to the aquarium. Do not perform partial water changes or add ammonia binders unless you are experiencing multiple fish deaths. Doing so will reduce ammonia toxicity but will prolong the cycle.
• A species of nitrifying bacteria, nitrosomonas, begins to convert the toxic ammonia into equally toxic nitrite (NO2).
• Another nitrifying bacteria, nitrobacter, then converts the toxic nitrite into relatively harmless nitrate (NO3). This completes the cycle, allowing you to add additional fish and commence regular, periodic water changes and gravel vacuuming.