How to Choose Aquarium Equipment

Aquarium Water pH Levels

Fish are picky about the water in which they swim.
© Peter Harrison

Understanding aquarium water pH levels is important, not only to have an idea of what it is but also to compare later with the tank water to judge how things are functioning in the tank. A simple aquarium water test kit will provide a reasonably accurate reading of the pH.

The pH scale runs from 0 to 14, and for monitoring aquarium water, you need to work in increments of tenths.


A pH value of 7 is the midpoint, which means the water is neutral. As the pH values go down from this midpoint, the water is increasingly acidic; as the value goes up from the midpoint, the water is increasingly alkaline.

A change of one whole number (i.e., 7.5 to 6.5) actually represents a change in acidity or alkalinity of 100 times.

Many aquarium fish that originate from South America prefer softer, more acidic water, whereas fish from East Africa do best in hard, alkaline water. These are just two examples.

Unless you intend to breed a species that is very particular about water chemistry, you will find that the stability of the pH in an aquarium is far more important than the exact value.

Large, rapid changes in pH are often fatal to fish. Any change greater than 0.2 in a 24-hour period will cause physical stress for most fish.

There are products on the market that can alter the pH up or down. However, changing the pH that quickly can cause the very problems you're trying to avoid, and the chemicals provide only a temporary solution. Once you use them, it can be difficult to maintain the proper pH without them. You would also have to store a supply of water with the altered pH for regular and emergency water changes.

Most fish will do fine in a broad range of values starting as low as 6.5 and going up to 8.0. Some species will do better at even higher or lower values than these, but for all practical purposes, it is best to allow the pH to settle at a value and simply leave it there.

As a natural part of the biological processes in an aquatic environment, the pH in the tank will become increasingly acidic over time. The change is very gradual, though, and so poses no real threat to the fish.

Eventually, the pH would drop low enough to cause problems, but the partial water changes you'll do as a part of regular tank maintenance will keep that from happening.

Learn about the differences between hard and soft aquarium water in the next section.

To learn more about freshwater aquariums, see: