How to Cycle Your Fish Tank

What is cycling?

To cycle, a fish tank basically means to establish, cultivate or grow a bacteria colony inside your filter. As complicated as it may sound, it really is as simple as waiting for bacteria to grow. Just think if you were to leave your toilet uncleaned for several weeks, imagine how much bacteria there would be inside the pan. Cycling a fish tank is basically the same process because you are relying on the toilet habits of your fish to encourage bacteria to grow in the media inside your filters. Unlike the lavatory in your house where you really don't want lots of horrible bacteria present, you are relying on all the bacteria inside your filter to actually keep your water clean and your fish alive.

The nitrogen cycle

I would encourage all fish keepers to gain an understanding of the nitrogen cycle as this will help you understand exactly what is going on inside your tank and how you can deal with water quality problems should they arise.

Do I need to cycle my aquarium?

The simple answer is yes, an aquarium must be cycled properly before you can safely add your fish. It doesn't matter whether the tank is 15 gallons or 500 gallons, it's still got to be cycled. If you were to simply fill your tank with water and then add all your fish at once then there would be such a massive buildup of ammonia, the chances are your fish would be dead within a few days.

Cycling your tank

Traditionally, there are two ways to cycle a fish tank. Both methods will involve introducing ammonia into the tank which will be the food the bacteria need to survive. The most common method of cycling an aquarium is to use small community fish that produce the ammonia themselves. A kinder, more acceptable way to cycle a fish tank is to use a method called the "fishless" cycle. This also involves adding ammonia to the aquarium, but as a name suggests you do not use live fish. In this article, we are going to use fish as it's probably easier for a beginner to undertake, and we wouldn't be happy with youngsters handling pure ammonia as it can be dangerous. If you would prefer not to use live fish then read this article on how to carry out a fishless cycle.

Which fish to use for the cycle

We would recommend that you use small community fish like the Barb. The Tiger and Cherry Barb are absolutely ideal as they are quite a hardy species of freshwater fish and unlike some more sensitive species, won't turn belly up as soon as they are exposed to ammonia. If you are cycling a very small tank less than 20 gallons then you are probably better off using much smaller fish like guppies or neon tetra. Your fish store should be able to give you advice based on what fish they sell.

It's important not to add too many fish as this will create a large ammonia spike very quickly which will probably just kill the fish within a few days. For a 55 gallon tank, 10 barbs would be appropriate. For a 75 gallon tank, you could go up to 15, for 100 gallons plus, you're looking around 20 upwards.

Seeding your new aquarium

It's become quite popular to kick start the cycling process by seeding your new aquarium with biological media that already contains live nitrifying bacteria.

So exactly what does this mean, and how do you go about doing it? Well, for a start you're going to need a source to obtain biological media that contains live bacteria. It's going to have to come from a filter that is still running on a fish tank. You cannot take media from a filter that has been switched off for a few hours because the bacteria will all be dead. Alternatively, ask a friend if they can spare a little bit of media from their filter. You can use sponge biological media to seed your new aquarium, or you can use ceramic type media that is often found in canister filters.

Some people take existing tank water, substrate and also rocks from existing aquariums that are already established, I really don't think this is going to make much of a difference, though. The really good healthy nitrifying bacteria is going to be found in biological media.

Also remember that you've literally got to take the media from one filter and put it straight into the other. If you leave it to dry out, or just leave it lying around for more than an hour then the chances are the bacteria will all perish.

Don't be under the misapprehension that a little bit of established media is going to half the cycling process, it won't. However, it will certainly give it a good kickstart.

During the cycling process

It takes anywhere between 6-8 weeks to cycle an aquarium using fish. Until there are sufficient bacteria in the filtration system ammonia will be present in the aquarium water. If the ammonia is left to rise to high levels then the fish will be poisoned. Having said that, you need the ammonia in order to feed the bacteria and help them grow and multiply. Therefore you need to keep the ammonia at such a level that still gives the bacteria the food they need, but won't kill the fish.

In order to monitor ammonia levels, you will need a water testing kit. I would recommend API (Aquarium Pharmaceuticals) as I found them to be the easiest to use. For the first week or two, you're only going to be checking for ammonia in the aquarium. I've always allowed the ammonia to reach around 0.5 ppm. I will then carry out a partial water change to reduce ammonia levels slightly. To be honest with you, it's going to be very difficult to accurately choose a level that you want to maintain the ammonia at, it's going to be a bit of trial and error. But what you've got to remember is that the bacteria need ammonia for their food, so you would be defeating the object by carrying out massive water changes which result in removing most of the ammonia. Maintaining ammonia levels between 0.25 ppm and 0.50 ppm will be enough to encourage bacteria growth, albeit a slow process.

After about a week into the cycling process, you could then move onto the next stage which is checking to see if nitrites are present in the filtration system. It would be helpful if you have a basic understanding of how the nitrogen cycle works. However, don't worry if you don't understand it, I'll try and explain as simply as I can what you are looking for. Basically, once you start seeing a reading for nitrite, you know that the bacteria that consume ammonia is now present and you are on the way. So for the next few weeks check ammonia and nitrite every two or three days and carry out partial water changes to keep them both at an acceptable level.

The cycling process is finished

After a few weeks, ammonia will drop to 0 ppm and nitrites will slowly drop as well. When both ammonia and nitrite are reading zero ppm you know that the tank is pretty much cycled and you can start thinking about adding fish that you are going to be keeping long term.

Add fish slowly

Don't be in too much hurry to add all your fish in one go. Remember that the filter is new and it wouldn't take a lot to overload it with ammonia. So it would be very unwise to go out and purchase the biggest Oscar you can find. If you are starting a new aquarium then start off with baby Oscars.

How do you know if your tank is cycled?

Sometimes it can be confusing and you really don't know whether the cycle is complete or not. There are a few tell-tale signs to look for that will put your mind at rest that your tank is properly cycled. If your tank can go for over a week without water changes but still maintain ammonia and nitrite levels at absolute zero ppm then you can pretty much rest assured that the tank is cycled properly. You should also be checking your nitrate levels on a regular basis now. The nitrate is a byproduct of ammonia and nitrite so if you have quite a bit of nitrate by the end of the week you know that the bacteria is doing the job properly. I think it's important that we mention nitrate testing for a moment. In order to get a reliable reading from your water test, it's absolutely vital that you carry out the test properly. You must read the instructions on the nitrate water test properly in order to carry out the process so that you get an accurate reading.

Cycling a fish tank can be frustrating for people who have never done it before and don't really know what to expect. The tank cycle is actually a natural occurrence, you've just got to keep an eye on things to keep your fish alive during this process.

If you are thinking about setting up a small aquarium then bear in mind that you may never be able to complete the cycle properly. Aquariums less than 10 gallons don't have enough water volume in order for them to stay stable. The filters don't contain much media and therefore it's difficult to maintain a balance where you are giving the bacteria enough food, but not polluting the water too much.

Fishless cycle - cycle your tank without using fish

If using fish to cycle your tank doesn't sit well with you then the alternative is to carry out a fishless cycle. The biggest advantage with carrying out a fishless cycle is there's no need for any water changes, you simply add liquid ammonia and let nature take its course. Also, because no living creatures are involved you can add much higher doses of ammonia than any fish would ever be able to withstand. Therefore the cycling process doesn't take as long as it would do if you are using fish. This article will take you through the process of cycling your tank without fish.