Water is chemically known as H2O meaning the one molecule of water is made up of two atoms of Hydrogen and one atom of Oxygen. However that isn't the whole story. Water has a natural tendancy to disassociate that is "split up" into what are called ions.
H2O = H+ + OH-
Water = Hydrogen ion + hydroxide ion
This is of interest to Fishkeepers because it is the amount of H+ ions present that determines how acid the water is. Water that has 10-7grammes per litre H+ is said to be neutral. As the amount of H+ ions increase then water becomes more acidic and conversely as it decreases then water becomes less acidic or more alkaline.
In order to measure acidity we use what is called the pH scale (notice capitalisation). Neutral water is pH 7 because it contains 10-7 grams per litre H+. If we were to have ten times more H+ in the water (10-6g/l) then it would be pH 6. Ten times less (10-8g/l) then it would be pH 8. You should notice two things about these figures: firstly as acidity increases so pH goes down and second, that each change in pH value (from pH 6 to pH 7 for example) represents a change in acidity by a factor of ten. So pH 4 is ten times more acidic than pH 5, one hundred times more acidic than pH 6 and one thousand times more acidic than pH 7. Because of this I'm sure you can now see why any changes to pH that we subject our fish to must be done very slowly.
pH effects all the inhabitants of your tank from the fishes right down to the micro-organisms and abrupt changes can cause some very serious problems. As we will see later pH effects practically all aspects of water chemistry and for this reason it is important to understand it. If you only ever buy one test kit then make it a pH test kit.
Most freshwater fishes are happiest with a pH in the range 6.0 to 7.5 but preferences for pH as low as 5.0 or as high as 9.0 are not unknown in certain species. Marine fish, on the other hand prefer pH 8.0 to 8.5 with 8.4 being considered normal.