Fish in general evolved in huge bodies of water, lakes, rivers and the oceans. Once we take them from those huge areas and place them in comparatively tiny tanks, a delicate balance is needed to keep the fish alive. Fish produce ammonia, which doesn't smell nice at all, ammonia is used as everything from a fertilizer, to a household cleaner, and whilst it leaves a streak free shine, it's extremely toxic to fish. But not to worry, there are bacteria that thrive on ammonia, converting it to Nitrites, still toxic, but again our friendly bacteria come to the rescue and convert this toxin to Nitrates. Whilst this is not healthy for your fish, they can stand a bit of it. You just need to do weekly water changes to dilute these Nitrates. Your filter does two things, firstly it filters out the solid wastes, uneaten food, fish pooh, and other lovely things. But it also allows the water to run over specialist surfaces where the bacteria survive, they will also live in the gravel or sand in your tank, and in the water. This is why small bowls and no filter equal a short life for your fish. But what causes you tank to start smelling, you could have had it months or years, and one day you notice the smell, it could be slightly pungent, all the way through to rotten eggs smell. The most likely cause is that delicate balance has been upset. Last week I opened the door to the fish house, and realised something was not right, the tank with a pair of breeding angels smelt awful. The cause turned out to be a faulty filter, some uneaten food, and the fact that I hadn't done a water change because of the eggs that they were guarding. A quick check of the filter and a water change, and removing all the uneaten food and the smell was gone, and the fish were much happier. So if you're getting or setting up a tank start as you mean to go on, that smell isn't just a nuisance, it's a sign that your tank is not healthy. Leave it how it is and you'll soon start to see your fishy friends rolling belly up, and adding to the smell. Firstly get the largest tank you can afford, second hand tanks can be a great deal, make sure you see it full of water, and you can afford a much larger tank, the larger the tank the more stable the Nitrate Cycle will be. Secondly tap water does not contain the bacteria you need. Fill this with dechlorinated water, chlorine is added to the water to kill bacteria, including the ones you need in the tank. Dechlorinator removes these toxins, which is healthier for the fish as well, chlorine will kill them as well. Thirdly you need to get hold of these bacteria, there are a number of ways to do this. My favourite is to seed off the new filter from another tank, easy enough when you have 20+ tanks, but if this is your first then ask around, either find a friend with a tank, or there are numerous groups around the country, many are on social networking sites, and are always willing to help new members, see if you can swap some clean filter pads or wool for some dirty ones, keep them wet and treat them like a new fish. Another option is to buy the bacteria from the fish store, normally sold in a little bottle you can dose the tank and kick start the process. You can also add one or two small fish, and do constant water changes to dilute the poison until the bacteria are established. For all these methods buy a test kit, that way you can keep an eye on the Ammonia and Nitrite levels and keep you fish safe. Gradually you can add more fish, but remember the larger the volume of water to the number of fish, the healthier the tank. I always keep my tanks under stocked, the fish are healthier, they breed more, and the tanks smell, of nothing. Make sure you have a large enough filter, the temperature is correct, and ideally have an air pump aerating the water. But as I said sometimes the tanks been healthy and smelling of nothing for months, and suddenly a stench starts up. Before grabbing the air freshener stop, mostly because air freshener can kill fish, but also because this needs solving. As a quick checklist...1. Make sure there is nothing dead in the tank, be it fish, snails, plants etc. 2. Ensure that there isn't uneaten food, it tends to pile up behind ornaments and in crevices. If this is the case you're over feeding, I feed mine twice a day however much they eat in about two minutes. If there is any food left I net it out, however you tend to get used to how much they need. The only exception is the nocturnal fish like the plecs, you tend to have to leave the food in for when they come out later 3. Check your filter, it may not be working, or just be slimed up, clean your filter gently once a week in tank water you've taken out. 4. Do a water change, not too much, many people talk about 50% or even higher water changes every day for delicate fish. This is great if you're an experienced fish keeper, and know what to look for. You should be taking out between 10%-20% water once a week. I have a little mark on the side of my tank at 20%. I drop the water to that level, and then top up with clean dechlorinated water. Huge water changes can shock the filter and kill the bacteria populations. 5. Make sure you're not using too many chemicals, and remember medication can kill the bacteria, if you need to use medication do frequent water changes to dilute the toxins. 6. Make sure the water isn't stagnant, make sure the filter is moving the surface of the water. You can do this so lifting the water so the outflow of an internal filter is just on the surface. It may also be possible to fit a spray bar. If needed add an air pump, or even another air pump. You may need to do a couple of small water changes for a few days until the smell goes away. Test your ammonia, nitrates and nitrates with a test kit, and check your fish, they may be gasping at the surface, flicking off solid objects, or other illnesses. Smelling is one of the things that can take away from the sheer beauty of your tank, but good maintenance, keeping it clean and healthy and you'll have a clean, clear and smell free fishy world.