Tank Cycling

from "So you want to start a marine tank" which is from ReefCentral, a great site.

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.

How to "Cycle" a new tank.

In order to "cycle" a new set-up, one must introduce a source of ammonia/um. In the past, this was usually done by adding some hardy fish (usually Damsels) to the new tank. Although this works very well in a bare tank (with outside filtration), it’s not recommended for two reasons:

• It is cruel to keep any fish, no matter how "hardy" in water containing near toxic levels of pollutants.
• Damsels are very aggressive, and once the tank has "cycled", they consider the whole tank as their exclusive territory. They will attack, and possibly kill, any new fish being introduced to the tank. To make matters worse, the rock in a typical reef tank makes it hard, if not totally impossible, to catch them when you eventually decide to replace them.

I would start the cycling by initially only adding the sand. Let it settle for 2-3 days, with all the circulation going. Then, add one or two pieces of base quality live rock, and one or two dead prawns or cocktail shrimps (the food type, not a dead one from a LFS - that could be carrying any number of diseases!). At this time, start testing the water for ammonia daily. You should notice an increase in the reading, and after a few days, the reading should stabilise, and then slowly start to drop (this could take 1-2 weeks). At that stage, start testing for nitrite as well, and keep on testing for ammonia. You should now notice an increase in nitrites, and a decrease of ammonia. After another week or so, the ammonia level should be fairly low, and the nitrite level should have reached it's peak.

Once the nitrite level has also started to drop, you can add the rest of the base quality live rock, and once both the ammonia and nitrite levels have become unreadable low, you can then add the good quality live rock (containing many interesting inverts). This could cause another ammonia & nitrite "spike", although it will be much smaller than the first. This is not critical if you’re patient - just let the tank settle for another month or two before adding fish. This should give the macro algae and small critters on the LR time to recover, and to multiply to sustainable levels in the LR, and to migrate into the sand. During this time, you should also add some live sand, and/or some live sand "starter kits" and "reef janitors". Also, don't be concerned with the initial algae growths - it's quite normal to have some diatom, cyano, and hair algae growing as the maturation progresses.

"Cycling" a new system with uncured live rock.

Introduce the LR after the tank's been up about a week, and use it to cycle the tank. Wait at least two months before introducing the first fish.

Advantages:

• You don't stress (or kill) any fish, and do not have to add fish that you will have to remove at a later stage.
• You don't need to buy fully cured LR. In fact, the cheaper, uncured LR causes a better cycle, and usually have more diverse life forms in/on them.
• You can introduce all the rock at once, do your "rockscaping", and have the tank completely set up when introducing the first fish.
• You will have amazing growth of whatever comes on the LR - with no fish to eat them, you will get all kinds of macro algae, featherduster worms, soft polyps, even some corals, growing.
• You will also enable the "small life", such as copepods and amphipods, to establish and multiply to such an extent that they should survive predation by your fish. Obviously, if you only have 2 or 3 of a specie to start off with, and they're eaten by a fish on the very first day, none will multiply after that.

Disadvantages:

• You will have to wait before you introduce any fish.
• Because the initial bio load was smaller, you will have to add your fish at a much slower tempo to avoid causing another ammonia/nitrite spike.
• You might have some algae growing on the LR. This is usually not a problem, though, as you start off with a nutrient poor set-up, which can be easily maintained, as there are no fish to feed. Also, the growing macro algae compete for nutrients, thereby restricting the growth of nuisance algae.