The measurement of water's acidity or alkalinity is called pH. A reading of 7 is termed neutral, with 1 being the most acidic and 14 being the most alkaline readings on the scale. The pH of most seawater hovers around 8, with 8.3 being considered safe for most marine species. In the open ocean, pH is relatively stable, but in aquariums it can change radically, to the detriment of your pets. This holds especially true for coral reef species, because they are adapted to an extremely pH stable environment. Water tends to become acidic due to the respiration of animals (they release carbon dioxide, which becomes carbonic acid). Certain substrates help to mitigate these changes, and these are particularly important in marine aquariums, where an acidic pH is to be avoided. Limestone, dolomite, and coral are very useful in buffering pH changes and in keeping the water on the alkaline side. Effective bacterial filtration is also important in maintaining a stable pH.

Freshwater fish generally do well at a pH near neutral, but many species do best at higher or lower ranges. Freshwater is much less stable than seawater, in terms of pH, and thus it is important to research each freshwater species carefully. Do not to attempt to keep animals from acidic waters with those requiring alkaline conditions.

The pH of aquarium water will change over time. An accumulation of waste products, uneaten food, and/or dead and decaying plants and animals will lower the pH, while the addition of certain rocks or substrates, such as coral, will cause the water to become more alkaline. Regular monitoring of pH is therefore essential. A number of test kits are available, most of which utilize a color-coded chart that is compared to a water sample that has been treated with a reagent. There are commercial preparations available that will lower or raise the pH of your aquarium, or help to set it at a specific level. While these can be useful in certain situations, it should be noted that the most important factor is to monitor the reason behind the pH changes in your aquarium. Merely correcting the pH without removing the underlying cause of the change will mask the problem and, in the end, will do more harm than good. Also, be sure to make all pH changes very gradually. An animal being maintained in an improper pH may die if suddenly switched to the correct pH without adequate time to adjust.

Fish and other animals that are stressed due to an inappropriate pH level may react by swimming rapidly about, or, in the case of amphibians, by attempting to leave the water. They will also be active at times when they normally would be resting. Eventually, they will become listless and die. Be sure to monitor your animals carefully and learn their normal behaviors so that you can detect such changes.