|Two elements of unacceptable water quality are contamination due to toxins and water at too high or too low a temperature. Healthy fish need water that is oxygenated, clean (free of toxins), the correct temperature and at the pH levels necessary for the particular species of fish. Regular water changes and close observation will guarantee healthy fish. Conditioning is necessary if city or municipal water is used. The two main elements water conditioners remove are chlorine and chloramine, as well as neutralizing dense metals. pH is the measurement of acidity or alkalinity of the water. A pH of 7 is the level most often desired for freshwater aquariums, but different species require more or less than 7 and this requirement must be researched. Too high a pH can produce toxins, too low can affect your fishes' gills. Know the level you need when purchasing your fish. Purchase a kit to test your pH weekly. Decomposed food and fish wastes produce ammonia, which is toxic for fish. The remedy is adequate filtration, combined with replacing 10 - 25 % of the water in the aquarium every two weeks. These changes will control nitrites, nitrates and ammonia. Ammonia needs to be checked every week, as a high level is a serious problem in your tank. When introducing new fish nitrite levels will also need to be checked. Oxygen levels should be kept between 5 and 7 ppm, as all fish need oxygen to survive: add an aerator or air pump if your tank needs increased oxygen. Your fish will also need adequate and . steady heat in the aquarium. Excessive heat reduces the necessary oxygen levels. Top quality heating is recommended and should always be monitored with a thermometer in the tank. Running your filtration a week or two before adding fish will allow the water to properly cycle. Another method for cycling is to add a bacterial agent after you have conditioned your water and let the filter run in your tank for at least 48 hours. After fish are added, monitor you nitrite levels every day for two weeks, then every week until your tank is completely cycled.