pH is a measure of the acidity and alkalinity of the water and is measured on a scale from 0 to 14 where pH 7 stands for neutral water, above 7 is basic (alkaline) and below 7 is acidic. The pH value is determined by whether the matter (eg minerals, organic matter) that has dissolved in the water is alkaline or acidic and by the amount of dissolved matter.
Different species live in nature in different kinds of water. In South America and Asia the water is usually acidic (pH below 7) to neutral (around 7), in West Africa relatively neutral (around 7), in Central America and the deep-water lakes in East Africa are alkaline (above 7).
Therefore, it is important that the fish in the aquarium have approximately the same water. If you want to find out more about pH, such as how to measure, or how to change the pH value, There is alot of other information about different water chemical conditions you have to think about. For example, it is very important to know the KH value of water.
For each fish species, we have specified an interval within which the pH value must be in order for this particular species to thrive. Of course, through slow habituation, many species can get used to surviving in water values that are outside this range, but it is definitely not recommended because it is partly a matter of animal cruelty and partly in the long run still risks taking an end with fright.
Another important factor regarding the pH value is that it is kept stable, ie it must not vary too much, large jumps in the pH value (even if the upper and lower limit is within the range) are very harmful for many fish species.