We get a lot of posts on this forum asking about how to correctly set up a tank, and still more from people who have sadly been given bad advice on tank setup by a pet shop and are seeing problems as a result. There used to be an article about fishless cycling that I tended to direct people to, but sadly this has now disappeared, so I thought I'd have a go at writing my own guide. What is the nitrogen cycle? In terms of fishkeeping, the nitrogen cycle is how fish waste is processed by the filter bacteria to keep the water clean and healthy for the fish. Fish excrete ammonia, which if left unprocessed will build up in the tank and lead to all sorts of health problems. Any dead and decaying animal and plant matter in the tank also releases ammonia. Nitrifying bacteria living in the filter media digest this ammonia and turn it into nitrite. However, nitrite is also highly dangerous to fish, so a second type of bacteria process the nitrite and turn it into nitrate, which is much less harmful. The level of nitrate in the tank is managed through water changes. Live plants also absorb a certain amount of nitrate from the water, helping to keep it clean. Why fishless cycling? In order for a colony of nitrifying bacteria to grow in your filter, they need access to a source of ammonia. Many pet shops will tell you to run your tank for a certain amount of time before adding fish. Unfortunately, this is a completely pointless exercise, as with no source of ammonia in the tank, no filter bacteria can develop. There are three ways to provide your filter with a source of ammonia: through adding live fish, through adding fishfood, or through adding pure ammonia. When you add fish to an uncycled tank, they begin excreting ammonia, which will eventually lead to the establishment of a colony of filter bacteria. This is called a fish-in cycle. However, this is a lengthy process, and in the meantime those fish are being exposed to ever-increasing levels of ammonia in the water. Certain species of fish, including most of those often recommended by pet shops as "starter" fish, are pretty tough and can take a fair amount of exposure to ammonia without showing any symptoms. But below the surface, their organs are taking the strain of the constant toxin exposure, and both their quality and quantity of life will ultimately be affected. On top of this, many fish traditionally thought of as "hardy" - the common goldfish and the neon tetra are two examples that spring to mind - have been so over-bred for the aquarium industry that they're now not nearly as hardy as they once were. Poor water quality lowers a fish's immunity, making it susceptible to a whole host of secondary problems, from whitespot (ich) to fungus to bacterial infections. So the likelihood is that, if you do a fish-in cycle, you will start seeing problems not too far down the line, resulting in a lot of stress and hard work (e.g. daily 50% water changes) to try to resolve the issues, and you'll probably experience some losses. A fishless cycle consists of adding a source of ammonia to a tank to establish a colony of nitrifying bacteria before adding fish. While this can be time consuming (up to two months) and you'll undoubtedly be tempted to add fish sooner, it's well worth the wait. Your fish will be healthy and live longer. You'll save money on medications and treatments. And, as an added bonus, once the cycle is complete you can fully stock your tank straight away, rather than adding just a few fish at a time. What do I need? In addition to a tank, a suitable filter complete with media, your choice of substrate & decor, and a heater if you're going tropical, there's a few additional things you'll need for a fishless cycle. - A water test kit that can test for ammonia, nitrite and nitrate. Liquid test kits are recommended as they're far more accurate than the paper strips, and accuracy is important here. The API Master Kit is a good one and contains all you'll need. - A source of ammonia. Some people use fish food, but this makes it very hard to measure how much ammonia you're adding, so I recommend using pure liquid ammonia. Make sure the brand you use contains only ammonia and water (no surfactants). Most fishkeepers use Jeyes Kleenoff brand. - A dosing syringe (1ml or 2ml is best). - Tapwater conditioner - look for one that removes both chlorine and chloramine. Seachem Prime is the most economical as you only need tiny amounts. If you know anyone else who keeps fish, it's worth asking them for a little piece of media out of their filter. This mature media will be established with bacteria that will seed your own colony, speeding up the process quite significantly. The bacteria can't survive out of water for long, so be sure to get it from their filter into yours in the shortest time possible, and NEVER expose any filter media to untreated tapwater, as the chlorine will kill the good bacteria. First steps Rinse your tank and decor. Rinse out the substrate as thoroughly as possible and add it to your tank. Add any plants/decor. Fill the tank with water and add dechlorinator. Set up your filter, add the media (separate sponges/baskets of media are better than cartridges if possible) and start it running. Add the heater, if using, set it to the required temperature (around 24-25 C for most tropical fish) and turn it on. If you can get hold of any mature media and add it to your filter, now's the time to do so. Starting the cycle First, you need to work out how much ammonia to add. While many guides recommend around 4 ppm (parts per million), I've found that 2 ppm is plenty for most tanks and will make for a quicker cycle. (The exception is if you're planning on overstocking your tank from the start, e.g. if you're going to be keeping African cichlids.) But how much ammonia do you need to add to make 2 ppm? To work this out, you need to know four things: - The concentration of your liquid ammonia. This should be stated on the bottle - it's usually around 10%. - The volume of your tank. To calculate this, multiply the length (cm) x width (cm) x water depth (cm) and divide by 1,000. This gives you the water volume in litres. Remember to allow for displacement by substrate and decor. - The existing ammonia level, in ppm. You can measure this using your test kit. At the start of the cycle, this should be 0. - The target ammonia level - as previously stated, this will normally be 2. To calculate the amount of ammonia you need to add, the easiest way is to use an online calculator, such as this one: https://www.tropicalfishforums.co.uk/index.php?page=ammonia_calculator. Measure out the ammonia using a dosing syringe and add it to the tank. Once the ammonia has had a chance to dissipate throughout the water, do another water test. It should be reading 2 ppm. If it's too low, you can add another drop or two, and remember this for the next time you come to dose! Your cycle is now underway... The ammonia phase This is the part of the cycle in which ammonia-processing bacteria are produced. It can take anything from a few days to a couple of weeks - faster if you've added some mature media, and typically slower in coldwater tanks. Do an ammonia test and a nitrite test every couple of days. When you see ammonia levels start to fall, top them back up to 2 ppm, and from then on test every day and top up ammonia as necessary. You should start to see nitrite levels rise at this point. When the ammonia level is falling from 2 to 0 ppm in 24 hours, the ammonia phase is complete and you're ready to start the nitrite phase. The nitrite phase In this part of the cycle, the second type of filter bacteria are growing - those that turn nitrite into nitrate. This phase is typically longer than the ammonia phase, usually lasting around 2-4 weeks. Continue to test ammonia and nitrite daily, and start testing for nitrate too. (Be sure to read the instructions on the test kit carefully - the nitrate test in the API kit won't work properly unless it's THOROUGHLY shaken.) The important thing to remember at this stage is that 1 ppm of ammonia converts into around 4 ppm of nitrite. So if you've been dosing ammonia for a while, your nitrite reading may well be off the charts. With this in mind, allow your ammonia levels to drop to 1 ppm, and maintain them at this level. Too much ammonia at this stage can stall the cycle. If this should happen - i.e. if you've been in the nitrite phase for over 4 weeks without seeing any drop in nitrite or rise in nitrate - do a 50% water change, then continue testing and dosing as normal. Once you do see a drop in nitrite, things should happen pretty fast. When you are able to dose 1 ppm of ammonia one day and get readings of 0 ppm for both ammonia and nitrite 24 hours later, your cycle is complete. Final preparations To be on the safe side, it's best to double check. Give it a few more days of dosing 1 ppm ammonia and checking after 24 hours. If the ammonia and nitrite have both fallen to 0 every time, you're nearly ready for fish! First you need to get rid of some of the excess nitrate generated during the cycling process, so do a 50% water change. Hopefully this should leave you with a nitrate level of under 40 ppm. (In some areas, tapwater contains nitrate at around 40ppm. In such places, it may be harder to reduce high nitrate levels, so more water changes might be needed. However, almost all fish are fine with levels around 40 ppm.) Don't forget to add dechlorinator to the fresh tapwater before you add it to the tank, otherwise you'll kill off the nitrifying bacteria and undo all your hard work! As a final check, add one last 1ppm dose of ammonia to the tank, and wait 24 hours. Test ammonia and nitrite to make sure they're still being reduced to 0. Congratulations - you're now ready for fish! Remember that the filter bacteria need a constant food source, so if you can't get fish right away, continue adding 1 ppm of ammonia to the tank daily until the day before you get the fish, remembering to check that the levels fall to 0 within 24 hours each time. Then, right before getting the fish, do another 50% water change. When you add the fish, continue doing water tests daily for a few days. If you should see an ammonia spike at any time, do a 50% water change, adding a whole-tank dose of Seachem Prime, as this helps to detoxify the ammonia. Continue testing and water-changing daily until ammonia and nitrite levels are stable at 0 again. But if you've followed this method, the likelihood of an ammonia spike is very slim Good luck, and feel free to ask on this forum if you need any help or advice at any stage!