Chlorine and Chloramine
The Danger Frequent weekly or bi-weekly water changes are an important part of keeping a fish tank healthy and stable. Over time waste products accumulate. And when water is lost to evaporation, the minerals and waste products are left behind, increasing their concentration over time. Also, the nitrification process (The filter converting ammonia into nitrite into nitrate) will slowly decrease the water's buffering capacity. If the buffering capacity is depleted, the pH can crash very quickly. Water changes help remove the bad stuff, and help replenish the tank water's buffering capacity.
When adding tap water to a tank for water changes, several things should be done to make sure that you don't shock the fish. First of all, the temperature of the replacement water should be close the same temperature as the tank water. Second, and just as important: The water must be chemically safe for the fish.
If you get your tap water from a commercial water system, that water is treated to make sure it's safe for human consumption. The water is cleaned, and filtered. Then, chemicals are added to the water to prevent anything harmful from growing in the water while it's in the pipe leading to your home. Until recently, most water treatment facilities used Chlorine to kill off any organisms in the water. The small dose of chlorine is safe to drink, but many people notice the slight chlorine odor. One problem water treatment plants have with chlorine is that it's unstable, and easily dissipated from the water. This means that the treatment plants need to put in higher levels of chlorine, so that they can be sure that some will remain in the water when it reaches your home. Recently, water systems have started treating tap water with chloramine instead of chlorine. Chloramine is a combination of chlorine and ammonia. It's much more stable than chlorine. It won't dissipate from the water as easily, and it isn't as likely to combine with other chemicals. But, chloramine isn't as good at killing off the microorganisms in the water as chlorine, so higher levels of chloramine are often used. Typically, water treatment plants use about 1 ppm of chloramine.
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