You should know if your tap water is hard or soft so you can adjust it if necessary for your aquarium. Hard water has a high content of certain minerals -- magnesium, calcium and iron salts -- and soft water does not.
The biggest concern with tap water, though, is what the city water department puts in it. Most municipal water companies add chlorine or chloramine to the water to kill certain bacteria that are harmful to people. Unfortunately, these chemicals are themselves harmful to fish and must be removed from the water.
Every pet store has a selection of chemicals that will easily dechlorinate the water. For dealing with chloramine, however, you need something a little different.
Chloramine is a combination of chlorine and ammonia. When a normal dechlorinator is used at double the recommended amount, it will break the bond between these two compounds, neutralizing the chlorine but releasing the ammonia. The added ammonia may be more than the nitrifying bacteria in your tank's biological filter can handle.
The only way to handle this problem is to use a one-step water conditioner designed to handle chloramine. This product will break the bond between chlorine and ammonia and neutralize both.
The easiest way to determine what your local water company is adding to the water is to call them and ask. Be aware, however, that cities using chlorine may suddenly switch to chloramine, which could cause problems if your tank doesn't have effective biological filtration or is overcrowded.
If you have well water, you might consider having it checked by a professional laboratory. Although there will be no chlorine or chloramine in the water, it may contain high levels of iron or other substances that could prove harmful to your fish.
In areas with farming, well water sometimes contains high levels of nitrates, which only add to the levels already in the tank from the nitrogen cycle. These nitrates come from the fertilizers used on farm crops.
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