The measure of the acidity or alkalinity of water is called pH. You will need to check the water's pH level when you first set up your aquarium. There are several varieties of pH testing kits that can be used. The pH levels required for freshwater and saltwater fish vary according to the species. Consult with your PETCO aquatics specialist to learn the appropriate level for your fish.

During the initial setup of your tank, the pH level should be checked every day. Once you know the water is within the accepted range, you should test twice per week to make sure the pH level is not fluctuating. Whatever the necessary level, it is important the pH remain constant. The pH level can be affected by several factors including overstocking your tank or overfeeding your fish.

If you need to adjust the pH of your water, buy a kit that includes chemicals to increase the pH and consult with your PETCO aquatics specialist. However, you will rarely need to adjust the pH levels in a healthy aquarium.