As soon as fish first move into your new aquarium, they will start to produce waste. Their waste contains ammonia.  Ammonia is highly toxic to fish, but is ultimately converted, by your filter, to nitrites (also toxic) and then those nitrites are converted to the much less harmful nitrates.  This is all thanks to beneficial bacteria which live in colonies in the tank’s filter.

Trouble is, your filter will not contain any of these bacteria, as it is new.  So, for the first few weeks, your fish will be swimming around in their own waste products and, if they manage to survive, will at best not be very happy, and at worst could become subject to irreversible physical damage, which could affect their growth and shorten their lives.  So, if you have already added a few fish, what can be done?

If you have friends or relatives with a healthy fresh water aquarium that has been up and running for ages, ask them if you can borrow some of the used filter media from inside their filter.  This will help to ‘kick-start’ the growth of beneficial bacteria in your own system, and, with very regular partial water changes (preferably daily), your fish will stand a chance of staying healthy.  A far preferable method of getting your tank up and running, before you even add fish, is that of the fishless cycle.  This is the method recommended by the RSPCA, and most experienced fish keepers.

How to do a fishless cycle, to provide a safe home for happy fish.  First buy a water testing kit.  A liquid test-kit is generally more accurate for this process than the kind that comes in the form of strips (though these can be handy once you’re up and running, to keep an eye on your water conditions every now and then).  These kits test for the presence of ammonia, nitrites and nitrates in your aquarium’s water.  You will also need a source of ammonia (as with no fish in the tank, there will be none occurring naturally, and therefore nothing for ammonia-eating bacteria to live on) which can take the form of fish food, which, as it rots, creates ammonia, or, most preferably, pure chemical ammonia (sold as a generic cleaning product, and available in our shop or through any hardware supplier).  What you will be doing, in a process that can take between two to six weeks, is building a bacteria colony in your filter, to ensure that all ammonia that comes to be present in the tank once the fish are added is converted to nitrates, which can then be reduced to a safe level for your fish via regular (as in every fortnight or so) water changes.

To start, add ammonia to 3ppm to the freshly added, dechlorinated tank water.  Leave the tank heater and filter running, and have the lights on for the period you plan to have them on for once you add your fish (8 hours a day is recommended).  Test the water for ammonia, nitrites and nitrates two days later, note down the results, and add more ammonia up to 3ppm if it has reduced.  Depending on the size of your tank, differing quantities of ammonia will be required, in order for it to be 3ppm.  You can find calculators online to work out the exact amount needed.   Repeat this process when ammonia reading drops significantly.

After several days, you should see a notable increase in nitrites alongside a pronounced drop in ammonia.  Congratulations: this means you have successfully grown your first batch of beneficial bacteria!  Carry on with the process, keeping the  ammonia topped up to 3ppm, and you will soon see the nitrite level drop and the nitrate level increase.  This means the second batch of beneficial bacteria (the ones that convert nitrites to nitrates) is ready and they are now doing their job properly!  Once you have done a partial water change to remove the build up of nitrates (the final product of the cycling process), your aquarium is now safe for fish, though you might want to continue with one more dose of ammonia and one more water test, just to make absolutely sure it is fully cycled. 

You should still add your fish with caution, as too many in one go could create a spike of ammonia:  test the water every few days in the early weeks to keep an eye on things, and be ready to do a water change if there is anything amiss.  You should find, though, that having been dosed regularly with pure ammonia during the cycling process, your filter’s bacteria are strong and healthy enough to be able to deal with anything your fish throw its way!