Water Quality: Sources of Water
Municipal/Tap Water: Municipal (tap) water can be a good option for use in a freshwater aquarium if some precaution is taken first. Most municipalities treat drinking water with either chlorine or chloramine for disinfection purposes. Chlorine is extremely toxic to fish and needs to be completely removed before the water comes in contact with fish. Chloramine is chlorine bonded to ammonia, both of which are detrimental to fish. While chlorine can be removed with aeration within about 24 hours, chloramine is much more stable and can be difficult to remove from water. Chlorine (and the chlorine portion of chloramine) can be removed with the simple addition of a commercial dechlorinator (follow manufacturer's instructions for treatment amount). While many dechlorinators remove either chlorine or chloramine, some remove only chlorine, so check the label first.
Well Water: Well water can vary greatly from location to location so it is important to consider the site. Well water usually has little or no oxygen but often contains high levels of dissolved carbon dioxide and nitrogen. As such, well water should always be aerated (approximately 24 hours) before coming in contact with fish. Some wells also contain high levels of hydrogen sulfide (which is detectible as a rotten egg odor) or iron, both of which are detrimental to fish. Heavy aeration will help remove hydrogen sulfide and will allow dissolved iron to settle out as rust, which can then be separated from the water. Well water tested and ruled as safe for drinking may or may not be suitable for fish, so additional water quality testing for fish-pertinent parameters (ammonia, nitrite, pH, alkalinity, hardness) should also be conducted. Simple commercial kits are available for testing all of these parameters. Well water may also contain other chemicals such as heavy metals or pesticides. There are treatments available that will bind up, or remove from solution, metals, and other pollutants. While well water is usually cheaper, doesn't contain chlorine or other additives found in municipal water, it doesn't go through the same strict testing. If in doubt, contact a water specialist in your area to test the water.
Reverse Osmosis (R.O.) Water: Reverse osmosis is a method by which water is forced through a semi-permeable membrane to remove many of the impurities (heavy metals, minerals, phosphates, nitrates and other dissolved solids) from the water. R.O. water is a good choice for make-up water from evaporation but not for initial startup or large water changes in freshwater systems, since by itself, it lacks many important ions that fish require for life.
R.O. filter systems are available for small or large scale, and are sized by gallons per day that they will produce. Small units (50 gallons per day) can be purchased that fit under a sink.
Softened Water: Softened water has had the hardness (mostly calcium and magnesium) replaced by the sodium ion. There are some plants and fish that come from waters with very low hardness (such as the Amazon rainforest) that will do better in soft water, but in most retail situations it is not necessary to soften the water.
If you do need to soften the water, the most common method is to use an ion exchange resin. Water passes through a resin that removes the hardness and releases sodium in exchange. After a period of time and use (determined by the hardness, amount of water softened, and size of the filter), the resin is bathed in a saturated salt solution (sodium chloride), which reverses the process, releasing the hardness ions, and picking up the sodium. This water is then discharged, and the filter is ready to work again.