Cycling the tank

8. Tank "Cycling"

To be honest, whenever I hear this term, I envisage this large, all glass tank, riding around on a bicycle …..

The term "cycling" is actually a bit of a misnomer, IMHO. In our hobby, it usually refers to a state where the aerobic component of the filtration system has "matured" to such an extent that there are enough aerobic bacteria present in the system to convert the ammonia/um generated by the tank's bio-load into nitrite, and the further conversion of the nitrite into nitrate. In actual fact, this only completes of the first phase of the actual nitrogen cycle. Until a few years ago, that was as far as the process could go, resulting in a slowly increasing level of nitrate, which could only be reduced by partial water changes. Thanks to the "discovery" of live rock, and later of live sand, the "'cycle" can now be completed by anoxic bacteria living in these substances, further converting the nitrate into inert nitrogen gas.

In hobbyist's terms, a new system is considered to be "cycling" when there are still detectable levels of ammonia/um and/or nitrite in the water. A system is considered to have "cycled" when these levels drop to zero, and the nitrate level starts to increase. The important thing to remember is that a tank does not "cycle" only once. The system will reach equilibrium, where there is enough aerobic bacteria living in the filters to maintain an undetectable level of ammonia and nitrite with a given bio-load. Any increase in the bio-load, such as when a new fish is added to the tank, will upset this equilibrium, by "flooding" the system with more ammonia/um than the bacteria can digest. Obviously, with more "food" available for these bacteria, they will multiply (assuming that food was the limiting factor), and in time the system will reach a new equilibrium, able to handle this increased bio-load. This does take time, though, and in the meantime the increased level of ammonia and/or nitrite could be poisoning all the fish.

Of course I did not know all of this when I started, and "Cycled" my UGF set-up with some locally caught cold-water fish. I did start off with a very light bio-load, though, and did not lose a single fish.