you set up a tank and you already have fish and then hear about the importance of cycling and wonder what to do now.
If it is at all possible you should return these fish to the store and then do a fishless cycle. I know you may be attached to your fish and may not want to part with them, so lets work on keeping them, however you must be prepared to accept that some of these fish may die over the next few weeks, you will need to do a lot of hard work to get through it………..Be prepared……… Buy a good water test kit. API Master test kit is one of the best, liquid tests give better reading than strips.

Still want to keep your fish? Cool, Do you know anyone with a mature filter? Is so ask them for  one third of their media in return for some new media materials. This will contain the bacteria you need to get your filter started,  help your fish survive and will take a lot of work out for you. Remember if you get some you must be careful transporting it, you should keep it in tank water and get it into your filter within 12 hours. If you do this you should still follow the instructions below, it’ll just be easier and quicker for you.

If you can’t get any mature media then you’re going to have to work harder. Firstly you need to look at what fish you have in the tank, you actually need to have a reasonable amount of fish to get your cycle going, I recommend about a quarter of the total stocking for the tank. The recommended stocking level is 1″ of fish per US gallon of water. So if you have a 20 gallon tank, you should eventually aim for 20″ of fish (not 1 20″ fish, but 20″ total combined from many small fish).To cycle the tank you need a quarter of this so 5″ of fish. Look up what fish you have, check what size they get to, add it all up and it should come to a quarter of the size of your tank is US gallons. This isnt exact so a rough estimate will suffice.
Once you’ve got the number of fish sorted out you need to test your water for pH, ammonia, nitrite and nitrate. Keep a log of the date you took the reading and what results you got. You will almost definitely get a reading for ammonia, possibly get one for nitrite and possibly for nitrate depending how long the tank has been running and what your tap water contains.

You can expect your results for ammonia to go up then down while the others stay relatively low, then when ammonia has almost dropped to 0 nitrite will start to rise and then fall, and nitrate will start creeping up. When ammonia and nitrite have dropped to 0 then you have a cycled filter.

So now every single day you should check your readings, if you get a reading for ammonia or nitrite that isn’t 0 you should carry out a 20% water change. This will most likely be every day for a few weeks; it can even take months in some cases. If you get some mature media you may find it’s done in a few days.

When you do water changes you should also vac the substrate and pick up any uneaten food or fish poo. However don’t be too thorough as some of the bacteria you need will live in your substrate and you don’t want to kill them off. Make sure you add dechlorinator to the water you put into the tank.

This will go on for a couple of weeks, it will be boring and hard work, but sadly it has to be done, doing it will save your fish.

Remember some medications can stop your tank cycling. So be extra vigilant for sick fish as you don’t want to be treating the full tank if you can avoid doing so.

Now eventually your readings for ammonia and nitrite will reach 0 and you can stop doing daily water changes. You should now cut back to between 10% and 25% water changes once a week. Please remember though, just because your filter is cycled it is only able to cope with the amount of waste your current fish produce, if you just get a whole load more the filter won’t cope and you’ll have to go through this all over again. From now on you should add fish very gradually, a couple at a time then keep testing the water, you may see a mini cycle in which case do your water changes again until it’s finished, then leave the tank a week or so and add some more fish. Very gradually over a few months your filter bacteria will build up and you’ll be able to finish stocking your tank.

Article written by Malakye;u=347