Hi all I've been going through the stickies and noticed we don't have our own version of the Nitrogen Cycle. So here it is. Introduction Time and time again, we come across people signing up and asking for help with their fish. Either the water is cloudy and they want to know what it is, or there's an actual health problem with the fish and the keeper is naturally worried. And every experienced keeper, trying to help, all ask the same questions of that fishkeeper. One of these is "How long has the tank been set up, and did you cycle it first?" At that point we're often met with the virtual equivalent of a blank stare. So. What is the cycle, and why is it so important we insist on asking about it? The cycle we refer to here is the nitrogen cycle, which you may have heard about briefly in primary school. Unfortunately that basic and rusty knowledge is not enough. Fishkeepers need to have a working knowledge of the nitrogen cycle in order to best help their fish thrive. Why it's Important Fish produce ammonia via their gills and in their waste products. Ammonia is toxic to them and, if left to accumulate, it's potentially lethal. It attacks the gills, causing the fish to struggle to get enough dissolved oxygen from the water, and causes massive internal bleeding, the sign of which is black marks in the fishes body, which are known as ammonia burns. Other signs include blood streaks in the fins and fin clamping, in which the fish holds its fins close to it's body. In later stages, the fish may become so oxygen-deprived that it's forced to gasp at the surface, or sit "panting" just above the substrate (gravel or sand). This is why filters are so important. They provide the optimum conditions to grow a set of bacteria that use ammonia as food, bringing ammonia down to, ideally, 0ppm. These first lot of bacteria give off waste of their own -, nitrite (NO2). Nitrite is equally as harmful to fish and must also be kept low - ideally 0ppm. Nitrite depletes the blood of oxygen, so those bright red streaks in the fish's fin rays will start to look brown. So that wonderful filter comes to the rescue again. It takes in the nitrite (NO2) and converts it into much safer Nitrate (NO3). Most fish can handle nitrate up to 40ppm, although 20ppm and under is ideal. Now, it's over to us. The filter can do no more. It's our job as fishkeepers to keep nitrate down, and we do that with regular partial water changes - at least 25% every week. Now, I can already hear some people shout out that their source (usually tap) water has nitrates at 40ppm. This is true. Some areas do have high nitrate levels. The addition of live plants can bring them down, as can using RO (Reverse Osmosis) water at a ratio of 50/50,. So, now you know what the nitrogen cycle is and why it's important, how do you cycle the tank? Well, the 2 most common methods are Fishless cycling and Fish-in-tank cycling. Whichever method you use, there are a couple of things you'll need. 1). A bottle of dechlorinator. There are plenty of different makes, ranging for £3 and up. Choose one that detoxifies chlorine and chloramine. The only time the brand becomes important is if you need to do a fish-in-tank cycle. 2). A master test kit such as the API or NT Labs. They'll set you back between £21 and £30 on eBay and are worth their weight in solid gold. Fishless cycling (recommended). Fishless cycling involves adding a source of ammonia to the tank and testing it every day until it starts to fall. The most wildly advised source of ammonia is household ammonia that you buy from the cleaning aisle. Add enough drops to bring the ammonia up to about 2-3ppm. Why that much? Because of the filter can handle 2-3ppm ammonia, it'll handle anything you can throw at it in terms of fish. Once it starts to fall, start testing for nitrite as well as ammonia. Allow ammonia to fall to 0.25ppm and then top it back up to 2ppm Nitrite will begin to rise until it reaches a peak - where it stays level for a few days to weeks. This is normal. As long as it isn't too high (over 5ppm), it will come down again. And when it does, it can be sudden. It's not unheard of for nitrite to drop from 2ppm to 0.5ppm or less overnight.. Meanwhile, you'll notice you're finding it harder and harder to keep the ammonia level at 2ppm. This is good. Those bacteria are hungry and converting it quicker and quicker. The tank is cycled when you can dose ammonia to 2ppm, test ammonia and nitrite 12hrs later, and get 0ppm for both. At this point, test nitrate, carry out a water change (about 50%) to bring it down, and get the fish. This process will take about 2-6 weeks on average. Increasing the temps to about 30°C will help to speed it up. Fish-in-tank Cycle "I've already got my fish. Are they going to die?" The short answer is, not necessarily, but the game changes. When you already have fish, the priority shifts from the bacteria to the fish themselves. You can't just let ammonia and nitrite rise unchecked. You'll still need a master test kit, as mentioned above. But now you'll need a bottle of Seachem Prime dechlorinator. As well as chlorine and chloramine, Prime also detoxifies ammonia and nitrite -, "locking" them away from the fish for up to 48hrs, while still allowing the the filter to carry out it's life-saving cycle. Cycling with the fish already in the tank involves the daily testing as above, but this time, there's no need to add ammonia as the fish do that for you just by living. In addition to testing every day, you'll need to carry out daily water changes of about 25% to keep the toxin level down. If either ammonia or nitrite reach 1ppm, even with daily water changes, do an additional water change. Adding live plants can also help by taking in some of the ammonia and oxygenating the water. Once again, the tank is cycled when you've seen ammonia and nitrite rise and fall back to 0, but this time, it's not as simple as just buying more fish. Now you'll need to quarantine any new stock. But that's an entirely different sticky.