Information on How to Cycle a Home Aquarium

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Cycling a fish tank isn't quite as complicated as you may think.  In fact when many of us started out keeping fish, we inadvertently cycled our aquariums without even realising what we were doing.  When I first got into fish I didn't have the luxury of the Internet where I could with the click of a button and the press of a few keys find all the information I needed.  I started out keeping fish like guppies and other small community fish.  I couldn't understand why all my fish kept dying for the first month or two, I would just keep replacing them as they turned belly up.  Then after a few weeks they miraculously stopped dying, I had in fact inadvertently successfully cycled my aquarium without even realising what I was doing.  Obviously for the sake of the fish and your pocket, you really want to try and avoid killing every fish you put in the tank during the cycling process.  Thankfully we now have the Internet so there really isn't any excuse not to do it properly. This page will explain in basic terms what cycling means and how you go about doing it.


The nitrogen cycle

The image to the right clearly shows how the nitrogen cycle works.  Fish produce ammonia through waste/respiration.  Beneficial bacteria take the ammonia and convert it into nitrites.  Then a slightly different type of beneficial bacteria take the nitrite and convert it into nitrate.  The diagram shows that nitrate will build up in your aquarium water, this is then reduced by the water changes.  However, plant matter will take nitrates as food as well.  Decaying and dead plant matter also produce ammonia, so it all starts over again.  That is a simple explanation of the nitrogen cycle. 



What Is Cycling?

Cycling a fish tank DOES NOT mean filling it with water and leaving it to stand for a week or two. it's very important that you know how to cycle a fish tank properly. Keeping fish in a fish tank is a huge compromise compared to what their life would be in the wild. Fish don't have to worry about ammonia or nitrite in their natural environment, they have millions and millions of gallons of water at their disposal.  Once you put a fish into an aquarium you have a completely different ball game altogether, you now have a situation where dangerous toxins can easily overcome an aquarium and kill all of your precious fish.  An aquarium that is properly maintained becomes a living environment that can sustain life for many years if maintained properly.

Before you add any fish to a tank, it is vitally important that you prepare it properly so that the fish have a suitable environment to live in. It is not just a case of filling a tank full of water and plonking your fish in it. People who are new to fish keeping often go out and buy an aquarium set up, fill it with water and then just add their fish. They then start wondering why the fish start dying within the week. Basically, the fish are being poisoned to death by their own waste. A basic complete aquarium setup consists of the aquarium itself, a lighting system, a heater, and a filter. The heater and lighting are self-explanatory. However, the filter is a little bit more complex than a lot of people actually realise. Newcomers to Aquatics tend to think that the filter is just for removing debris from the tank. Yes, the filter does this, but that is only half of the story.

Your filter is actually the heart of your tank, without it, your fish will not survive. Whereas a brand new filter will remove solid waste, it will not remove dangerous toxins. The filtration system will only do this once you have the correct bacteria living within it. Growing bacteria in a new filtration system is commonly known as "cycling".

When the fish goes to the toilet, ammonia is produced (NH3, NH4). In a tank that has been cycle properly, the nitrifying bacteria that inhabit the filtration system turns ammonia into nitrite (NO2) which is then turned into nitrate (NO3).

So the term "cycling a fish tank" simply means establishing a bacteria colony in your filter so that fish will stay healthy and not die from toxic poisoning.


Freshwater Testing Kit

API freshwaterIn order for you to cycle your aquarium properly you will need to test the water on a daily basis using a special water testing kit. You can buy the individual solutions separately. However since you will be testing for ammonia, nitrite, nitrates and pH, you're probably better off buying a master test kit which comprises of all the necessary solutions you need. These test kits also come with four glass test tubes, easy to read colour chart and instructions. I have used API products for many years and in my opinion you cannot get a better quality water testing product. 


Cycling a Fish Tank

Traditionally there are two ways to cycle a fish tank. The most common way is to use fish as a source of your ammonia. The other way is to carry out a "fishless cycle". As the name suggests you don't use fish as a source of ammonia, you use liquid ammonia which can be found in household cleaning products.

If you are changing aquariums then you can use existing biological media from the filter that is running on the old tank, this biological media will already have healthy bacteria living on it. Obviously it very much depends on how much media you have at your disposal, but even a small amount will be enough to jumpstart the new filtration system. When we take existing media from an old filter to add to a new, we call this "seedling" the new filter. Just remember to keep the old media wet while you are changing aquariums. If the process of changing aquariums is going to take a while then you could help your beneficial bacteria by placing an air stone in with the media, this will oxygenate the bacteria and help keep them alive and healthy. If you are starting from scratch with a completely new aquarium filtration system then the whole process of cycling will take several weeks to complete if you are using fish as a source of ammonia. Cycling your tank using liquid ammonia can cut this period down considerably to just two or three weeks.

Some people like to use existing tank water, substrate, rocks and decor to help seed the new tank. Even though there won't be an enormous amount of bacteria present in the water and substrate, there will be some and every little bit counts.

You can use any freshwater tropical fish you like to cycle and aquarium, as long as you have a source of ammonia, the cycling process will be the same. However, you have got to remember that during the cycling process, the fish will be put under a lot of stress due to the amount of ammonia they will be exposed to. Therefore do not use Oscars to cycle your aquarium as you could damage their immune system. Even though they may survive the cycling, they could start running into health problems later on in life. It sounds a little bit mercenary but you are better off using fish that you are not going to keep long-term. You can use such fish as Danios, Barbs, Tetras, Guppies, Convicts, Plecos, Mollies, Firemouths. These are just a few that I know our members have used successfully. However, everybody will tell you that if you can get hold of some pre-seeded media then you will definitely aid the cycling process. Just be aware that you will be exposing these fish to high levels of ammonia so don't get too upset if you do have a few fatalities. I better mention goldfish while we are talking about what fish to use for cycling. There is a little bit of controversy to whether or not goldfish should be used to cycle a tropical aquarium. Some people suggest that because they are cold water they shouldn't be used. Other people are worried that because goldfish are produced in massive numbers, they can often harvest disease which can be passed onto your new aquarium. Having said this, some of our members have successfully cycled their tank using goldfish so that is something for you to consider. All I would say is that if you are going to use goldfish, source them from a reputable shop that can guarantee they are disease-free and healthy.

One important thing to remember is you will need some water quality testing kits that you will need to use every couple of days during the cycling process. Either this or get your local fish store to test your water for you. My advice would be to learn how to test the water yourself, it's very easy and isn't complicated at all. Knowing how to test your aquarium water yourself is really very important if you are serious about keeping tropical fish, especially Oscar fish.

If you are cycling a large aquarium then you may find it takes a day or two, maybe longer before the ammonia rises to a level that water changes need to be carried out. To be perfectly honest with you, it's very difficult for me to predict exactly when the ammonia in your particular tank rises to a level that the water will need to be changed.

If the ammonia is under 0.25 then you are okay. If it's between 0.25 & 0.5 then carry out a sizable water change, I would say no more than 50%. If your ammonia is very high, 1.0 and above, carry out a very large water change, 75%, 80%. Many people do a water test directly after the water change. Sometimes you can get false readings doing it this way so always carry out your water tests before your water change. It's not worth panicking about, your water change should remove the ammonia so don't worry.

I'm not going to commit myself and say how long it will be before ammonia returns to zero so don't worry yourself if the ammonia remains present for a while. At some stage you will test your water and the ammonia will be 0. Then if everything runs smoothly, you will notice an increase in your nitrite level. Once again, your nitrite will eventually drop down to 0. you will eventually start seeing a nitrate reading. It may be a case that nitrate becomes present at a much earlier stage, however concentrate on your ammonia and nitrite until they remain stable at zero.  Nitrate is a byproduct of ammonia and nitrite and is not toxic like ammonia or nitrite. Once your ammonia and nitrite are zero, you have established a biological filter and your tank is cycled. The whole process could take six or seven weeks so be patient. Try not to panic during the cycling process if one day your ammonia is higher than the previous day, if you just carry on monitoring the ammonia and nitrite and do water changes accordingly, eventually the bacteria in your filtration will buildup to the extent that you will not have any ammonia or nitrite present, but you have to remember it will take several weeks if you are just using a few small community fish, so be patient. Do not clean your filters during the cycling process, unless you have some very fine polishing pads that get clogged up very quickly.

When your aquarium is showing signs of being fully cycled, i.e. ammonia and nitrite are reading 0, and you have a decent amount of nitrate being produced, you can then start adding your fish. One important thing to remember is that a newly cycled tank will not be completely stable. Don't be tempted to go out and buy a tank full of fish straightaway. Introduce fish gradually and let your biological filter catch up with the bio load that the fish produce. You're much better off doing it gradually than risking running into problems by adding too much too quickly.

Cycling an aquarium can be both lengthy and frustrating at the best of times. If you are doing it for the first time then you may well be pulling your hair out after the first two or three weeks. The best advice I can give you is just stick with it and follow our instructions, eventually all will come good, I guarantee it :-)

So to reiterate on what we have learned:

  • You must cycle your aquarium before fish can live in it and survive long-term
  • You can either use community fish, or pure ammonia from a bottle to cycle your aquarium
  • If possible use biological filter media, gravel and water from a cycled tank. 
  • Test your water every day so as to maintain a safe ammonia/nitrite level 
  • Add fish gradually once your tank is cycled to avoid excess ammonia building up

cycling an aquarium-Tiger Barbcycle an aquarium with Mollycycle aquarium with convicts fishcycle aquarium with fire mouthcycle aquarium with giant Daniocycle aquarium with Tetracycle aquarium with guppy fishcycle aquarium with pleco

Pictured above are several fish that make good candidates for cycling an aquarium. Pictured from top left to right:

Tiger Barb, Molly, Convict Cichlid, Firemouth, Giant Danio, Neon Tetra, Guppy, Common Plecostomus


API - Aquarium Pharmaceuticals

I have used various water testing kits over the years, Tetra and Hagan to name a couple. However, my favorite water tests are manufactured by a company called Aquarium Pharmaceuticals. I use their liquid water testing products such as the ammonia, nitrites, nitrate and pH tests. You can buy them individually or all of them together if you go for one of their master testing kits. The next best one in my opinion are manufactured by Tetra, although I have found these to be a little bit more expensive.


Bacteria In a bottle

There are various products on the market that supposedly contain the bacteria that remove harmful toxins such as ammonia and nitrite. They claim that when you add these products to an uncycled aquarium, they instantly establish biological activity to the filter. I now quote a company that sells these products "Enables rapid stocking of new aquariums by instantly establishing the biological filter" you'll noticed it says establishing the biological filter. The nitrifying bacteria that is present in your filtration system requires oxygen to stay alive. They also need a surface to cling to, so to speak. They are not free swimming like some types of bacteria. So if they are not the type of bacteria that are free swimming, and they need a surface area to cling to, i.e. your media whether it be sponges or little ceramic balls, how do they stay alive inside the bottle, or, have they used another bacteria? There are other bacteria that are present in a fish tank. These bacteria do break down such things as food, plant debris and dead fish. However, they are not the same bacteria that you find in your filtration system. So, if they haven't used the nitrifying bacteria in the bottles, how can it establish a bacteria colony in your filtration system?

Having said all this, this stuff does work. I haven't used it on a large aquarium with Oscars, only on a 20 gallon community tank with a few cory catfish. I set the aquarium up and introduced two Cory catfish. I then followed the instructions on the stress zyme bottle, add 10 mills to every 10 US gallons. That worked out slightly less in the UK. I had to do this on the 1st, 7th and 14th day. Intermittent water tests showed excellent results of no toxins present. Approximately 5 weeks after setting the aquarium up, I carried out a water test, ammonia, nitrite and nitrate. Ammonia and nitrite 0, nitrate in between zero and five. So, we have to conclude that this product does work because I have done no water changes since setting the tank up and I have had no ammonia or nitrite readings at all. And we have nitrate reading which means that ammonia and nitrite are being dealt with by the bacteria. I don't think you should let it go to your head, the bacteria colony will be newly established and will not be able to handle a large bio straight off. Add fish gradually and keep testing the water.

As mentioned at the beginning of this page, you can always do a fishless cycle. A fishless cycle is exactly how it sounds. You cycle the tank without fish. What you do is use pure ammonia. This is often found in hardware stores. Having said that, it isn't always easy to come by so you may have to look around. A fishless cycle is a lot easier and more convenient than cycling the tank with fish. For a start, you don't have to do any water changes and more importantly, there aren't any fish that are put at risk from high stress levels that would be caused by poor water conditions. For a more comprehensive look at a fishless cycle, visit this website

How to do a Fishless Cycle...