Water should be infused with oxygen between five ppm and seven ppm if you want fish to remain healthy. The water should be aerated immediately if the level is below two ppm. To ensure that your fish are breathing enough oxygen, you can measure oxygen levels with a dissolved oxygen meter. The skin of fish and around their eyes will develop bubbles if they are exposed to too much oxygen. Depending on the species of fish, there is a different level of oxygen in the water that poses a danger to them.  It is generally recommended that you add 1 gallon of air per gallon of water to your tank. You can easily adjust the air output of an air pump to meet your needs.