The Essential Nitrogen Cycle - In Aquariums

by Tim

Any fish tank, large or small, needs some way to control the waste products of your beloved fish. This can be done by several methods, from changing the water every day to expensive canister filters and sumps.

Whatever you use in your tank, it is best to have some sort of filtration, purely to help reduce stress on the fish and yourself! Let’s look at the most common method of controlling waste, in the form of a filter.

There are several categories of filters available, depending on the size of the tank and the types of fish you keep. But they all have one thing in common:

Removal of ammonia and nitrites which can kill your fish.

The way this is done is by supporting a bacteria culture in the filter media and pulling water through it. This media can be many different materials, from simple floss to ceramic rings, or even standard aquarium gravel! The material isn’t important, so long as it is there and works!

That’s a very basic outline of how a filter works, but this isn’t to discuss the details of filters, but to help you to start and maintain a healthy tank.

The Nitrogen Cycle: What is it?

Simply, it’s a way of starting a fish tank. Your fish produce waste in the form of ammonia. This is very toxic to them and will kill them very quickly if allowed to build up.

The way the cycle works, is to convert ammonia into less toxic nitrites and then into even less harmful nitrates using two different types of bacteria held in the filtration system. Because this is a natural process, you can’t hurry it, and it will take a while for the culture to grow large enough to do any good. Once again, there are choices as to how to start this cycle off:

Fishless cycle:

A very good method to start your tank. This method involves no potential risk to fish, since they are not used. The way this works is to "feed" the tank with either pure ammonia or small amounts of fish food every day. Using test kits, readily available from LFS’s you will be able to track the progress of the cycle and determine when the tank is safe. The time it takes is dependant on the size of the tank, the type of filter and the amount of pollutants in the water. Don’t be tempted to put more in to speed up the cycle, it doesn’t work. It just wastes more of the ingredients you use. You will need to test the tank on day 4, then at least twice a week after that.

A 10G tank will normally take 2-3 weeks to cycle, a 55G tank anything from 5-8 weeks. You will know when the cycle has completed by reading the levels of ammonia, nitrite and nitrate in the water. When the first 2 readings hit 0 and the nitrate reading levels at around 10, your tank is ready for fish. Be careful though, to only add a couple at a time, otherwise the readings will start climbing again.

Cycling your tank using fish:

This is another way to cycle your tank, but you need to be careful which fish you use, and their size matters too. The best fish for this job in my experience is goldfish or danios. You can get these cheaply from your LFS and some stores keep fish specially for the purpose on a loan and return basis.

You only need a couple of small fish for this. Any more and you’ll lose some during the cycle because the ammonia levels will build too quickly. Remember, any level of ammonia above 0.1PPM is lethal. You will need to keep an eye on the fish during the cycle, especially the first week, to make sure that the levels don’t get too high for them. If they do, then a 50% water change is the only safe way to reduce the levels quickly, which also prolongs the end of the cycle. Testing the levels every day can get expensive too, as test kits are not cheap! (Fishless cycling makes sense now, doesn’t it?)

Using an existing Filter:

My preferred method. If you already have a cycled tank, then adding another filter and overfeeding the fish for a week or so will allow the filter to colonize with the bacteria needed. Then you just set up your new tank, add the pre-loaded filter and a couple of fish. The tank will then perform a mini-cycle, which lasts from a couple of days to a couple of weeks, depending on tank size and the amount of bacteria that is in the filter, but at no time should the levels of toxins ever threaten your fish. Regular water changes will help the tank to stabilize before you add more fish.

Whichever method you use, it is in your best interests to do this right. It will save you losing precious, valuable, beautiful fish!

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