What do you need to know about water chemistry and why? Water in nature is rarely pure in the "distilled water" sense; it contains dissolved salts, buffers, nutrients, etc., water treatment chemicals with exact concentrations dependent on local conditions. Fish (and plants) have evolved over millions of years to the specific water conditions in their native habitats and may be unable to survice in significantly different environments. Beginners (and especially the lazy) should take the easy approach of selecting fish whose needs match the qualities of their normal tap water. Alternatively, an advanced (and energetic!) aquarist can change the water characteristics to match the fish's needs, though doing so is almomst always more difficult than first appears. In either case, you need to know enough about water chemistry to ensure that the water in your tank has the right properties for the fish you are keeping.pH refers to water being either an acid, base, or neither (neutral). A pH of 7 is said to be neutral, pH's below 7 are "acidic" and pH's above 7 are "basic" or "alkaline". Like the Richter scale used to measure earthquakes, the pH scale is logarithmic. A pH of 5.5 is 10 times more acidic than water at a pH of 6.5. Thus, boiler water treatment chemicals changing the pH by a small amount (suddenly) is more of a chemical change (and more stressful to fish!) than might first appear. To a fishkeeper, two aspects of pH are important. First, rapid changes in pH are stressful to fish and should be avoided. Changing the pH by more than .3 units per day is known to stress fish. Thus, you want the pH of your tank to remain constant and stable over the long haul. Second, fish have adapted to thrive in a (sometimes narrow) pH range. You want to be sure that your tank's pH matches the specific requirements of the fish you are keeping.