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Until the water clears, change up to 20 percent of it on a daily basis. Remove some of the dirty water and immediately replace it with fresh water.  Do not change more than 20 percent of the water in one day. Changing more can shock you fish and cause them to become ill or die. Changing out some of the water removes at least some of the waste. The bacteria thriving in your water then begin to starve, thereby creating cleaner water. After the water clears, you only need to change water once every two weeks or so. If your fish do not regularly eat all of the food you put in the tank, reduce the amount of food by 5 to 10 percent.  When fish food breaks down, its particles can make the water dirty. The decayed food can also breed bacteria. Check on the aquarium 10 minutes or so after you feed your fish. If the fish have stopped eating at that point, use a net to remove any uneaten food. Too many chemicals can hurt your fish, but regular doses of aquarium salt, water conditioner, or quick cure can help clear your water. These chemicals are formulated to clear debris and correct bacteria levels.  Follow the dosage on your box of aquarium salt and apply the salt once. Follow the dosage on the bottle of conditioner and apply it each day until te water clears. Use half a dose of quick cure each day until the water clears. Only use one of these chemicals at a time. Mixing chemicals can hurt the ecosystem of your aquarium. Sprinkle a handful of gravel from an established aquarium into your tank. Wait roughly 24 to 48 hours and note the results.  Established gravel has beneficial bacteria on it. Unlike the harmful bacteria living in your tank, this bacteria can digest waste and balance out its harmful cousins. Beneficial bacteria also helps to establish a bacteria colony in your filter. When a colony grows in your filter, it attracts and draws bacteria out of the tank itself, clearing up the water. The gravel must come from an established tank with health fish. You can usually purchase some from a store that sells fish if you do not know anyone who can give you some for free. The aquarium must have a power filter, and that filter must be clean and working properly.  Feel the outside of the filter. If it seems abnormally warm, it might be clogged or damaged. Check the instructions for your particular filter model to determine the proper way to clean it.
Change some of the water. Reduce the amount of food. Add aquarium chemicals. Mix in some gravel. Check the filter.