Frequently asked questions on fishless cycling


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What is fishless cycling and why can it be a better technique to use than other methods? Matt Clarke answers some of the most frequently asked questions.

What is fishless cycling?

Fishless cycling is a method of maturing (or cycling) a new biological filter using chemical means rather than adding fish.

Maturation, or cycling, is simply the process of growing beneficial bacteria on the filter. You do this by providing a pollution source for the bacteria to use for energy, and then waiting several weeks for the bacterial colony to reach full size. When all of the bacteria have grown the tank will be free of ammonia and nitrite, nitrate will have started to build up, and the filter is termed mature, or cycled.

Why do you need to mature a new biological filter?

New filters do not contain the bacteria necessary to remove ammonia produced by fish and rotting food, so they are initially unable to keep the water free of pollution. The process is essential to ensure that the filter works correctly by keeping the water free of pollution.

I have been told I can leave my tank empty for a month to mature it. Is this right?

No. It's an old wive's tale. Before the maturation process can start, the bacteria need a source of nutrients. A tank that's left empty, or doesn't get a regular supply of pollution won't mature, and a mature tank that's left empty could potentially "unmature" if there are too few nutrients available for an extended period of time. You need to add a nitrogen source to the tank for the bacteria to use for energy.

My shop has advised me to add fish after a week. What do you say?

There are quite a few different ways to get a new filter going. Historically, fishkeepers have matured a new filter by this technique of stocking slowly with "hardy fish" after the tank has settled for anything from a day to a week.

Practical Fishkeeping has advised against using this technique for many years, and most forums you may visit will probably echo our sentiments. However, quite a lot of retailers still advocate this approach, so your shop may suggest you do it this way.

How does cycling with fish work?

The fish produce ammonia which builds up in the water and encourages the growth of the bacteria. However, as there are very few bacteria present when the filter is new the water will inevitably contain some pollution and this could irritate or harm the fish.

At first ammonia will be a problem, but then after a couple of weeks, nitrite levels will start to peak. The more pollution going in (whether it's from too many fish or too much food) the higher the ammonia and nitrite level will go, and the more likely the fish are to suffer from poisoning.

If you really must mature a new filter in this manner you need to be very patient and very careful. If you add too many fish or feed them too much you'll certainly run into problems. Some fish are hardier than others, so certain species can quickly succumb to disease or die.

If your shop has given you this advice and you've already followed it, test regularly and be prepared to make a drastic water change if conditions become dangerous.

Ideally, you should adopt the more modern fishless cycling technique, which is widely considered more ethical since it does not expose fish to toxins.

What are the benefits of fishless cycling?

Unlike the usual method, fishless cycling allows you to mature the system without placing fish under stress from polluted water. It means you won't have to choose hardy fish that you might not otherwise want to keep. And, if done correctly, it could allow you to reach a higher stocking density quicker and more safely than the conventional method.

Are there any disadvantages?

In most cases you won't be able to put any fish in the tank for at least three or four weeks, sometimes a little longer. Some experiments have shown that tanks cycled using chemicals, rather than organic materials like fish wastes or fish food, may lack all of the required beneficial bacteria and have to undergo a further "maturation" (or time lag) when fish are added.

Is fishless cycling a new technique?

No. The term "fishless cycling" is new but the actual technique has been around since at least the 1970's. One form of fishless cycling, overcompensation, is a widely used technique outside the aquarium hobby, but has only recently started to catch on with fishkeepers.

How do I get started?

Most people opt for an inorganic fishless cycling technique. This basically involves adding a nitrogenous solution, like ammonia, to the tank at regular intervals for the bacteria to break down. Arguably, organic material, such as a piece of fish or some fish food would be the ideal thing to use, since it would also encourage the growth of the heterotrophic bacteria that break down organic matter into ammonia. These might not be present in the same numbers if you cycle inorganically.

Liquid or crystalline forms of ammonia are the most commonly used chemicals for fishless cycling. However, they can be dangerous to handle, and could be tricky to get hold of - chemists and DIY stores sometimes stock ammonia.

We suggest using Waterlife's Biomature. Unlike most other products for cycling new tanks this one contains a source of nitrogen to feed the filter. It's not designed to make the tank mature any quicker, but you could add a bacterial supplement, or ideally some media from a mature filter to give the filter a kick-start.

The food source needs to be added at regular intervals throughout the cycling process.

What happens at first?

If you are cycling organically, by adding a bit of rotting fish, or feeding the empty tank with a pinch of flake food every day, the maturation process will be slightly different to that in a tank that's cycled with chemicals.

The first step (which many fishkeepers are unaware of) is mineralisation. This is the process by which ammonia is formed by heterotrophic bacteria, or by chemical processes (such as deamination) in the tank. Without mineralisation there will be little ammonia for the first bacteria to use. Experiments have shown that this may be why it takes a day or so for the rotting stuff to have an effect on pollution levels, and why the tank won't start to mature straight away.

How is the ammonia broken down?

Initially, the ammonia level will rocket, as there are few bacteria present to break it down. After a few days, autotrophic bacteria called Nitrosomonas start to grow. These nitrifying bacteria break down the nitrogen in the ammonia by oxidising it to nitrite. Nitrite is less harmful, but would still be very dangerous to any fish present. The ammonia level will normally remain very high for a couple of weeks in tropical aquaria, although it could persist for much longer if the water is cold, salty or contains chemicals that affect the bacteria.

What breaks down the nitrite?

The nitrite is broken down by Nitrobacter and Nitrospira bactera into nitrate, which is much less toxic. The most popular method for reducing nitrate is by changing the water. By swapping some of the polluted tank water for dechlorinated tapwater you'll decrease the nitrate level by dilution.

The nitrite level is getting high. When will it go down?

After about a fortnight (depending on the temperature) the nitrite level will have started to peak, and the ammonia level may have started to drop as the Nitrosomonas population has been increasing massively.

Keep testing the pollution levels and you should spot a point when they start to drop (after about week three in most tanks). If there is still some ammonia present, or if the water is cold, the nitrite level may stay for longer. High ammonia inhibits the growth of the Nitrobacter, so the nitrite won't be converted to nitrate until the ammonia level drops a bit lower.

Why is my water cloudy?

The population of bacteria multiplies rapidly during the early stages in order to try and make use of all of the nutrients in the water. Some experiments have shown that there can be as many as 100 million bacteria per gram of filter sand after just a fortnight! Add that up for a whole filter and you're talking squillions.

What's over-compensation?

Over-compensation involves adding much more pollution to the tank than the fish will create when the tank is fully stocked.

Ordinarily, each time fish are added to an aquarium the bacteria need to multiply rapidly to utilise the extra wastes. This can lead to temporary blips, called time lags, where ammonia and nitrite levels are raised for a few days while the bacteria multiply. If you keep sensitive fish, this could stress them. If you exceed the maximum amount of fish the filter can support (the carrying capacity) the "blip" will persist and you will find it virtually impossible to maintain ammonia- and nitrite-free water.

Overcompensating a filter can allow you to stock it with lots of fish in one go. I have used the technique to mature commercial filtration systems so that they are ready to take large shipments of fish in one go. It is also a very useful approach to adopt if you want to stock a mbuna system in one fell swoop.

Can you overdo it?

Yes. Adding too much ammonia will mean that it takes longer for the bacteria break it down. Nitrobacter are inhibited by ammonia. And you could add so much that you exceed the carrying capacity of the filter.

How can I speed up the maturation process?

Controversial research undertaken on many aquarium products for accelerating the speed of maturation have suggested that they are not particularly effective. Some have claimed that they could be based on the wrong types of bacteria. They're still worth a try we reckon and we've had good results with some of them.

The quickest way to boost a new filter is to add mature media from another filter. You need to ensure that the water chemistry is similar for best results.

Why do some tanks take longer to mature than others?

Filter bacteria multiply at different rates depending on the water conditions and the environment within the filter. Some things can repress their growth, while others still allow them to grow but slow their metabolism, making them less effective at oxidising wastes like ammonia and nitrite.

Many fish disease treatments (especially methylene blue, nifurpirinol and antibiotics) will cause problems. Chlorine and chloramine (from raw tapwater) are also things to look out for.

Does temperature matter?

Yes. Temperature is very important. The bacteria grow quicker when the water is warmer (so ponds take longer to mature) and drops in temperature can slow the process. In experiments in marine tanks, a rise of 4C increased ammonia oxidation by 50% and nitrite oxidation by 12%. Dropping the temperature by just 1C reduced ammonia oxidation by 30%.

This item was first published in Practical Fishkeeping in 2002. It was updated in 2010. It may not be reproduced without written permission.