PH
pH is the measure of acid or alkaline in your water. The proper pH is necessary for your fish to be healthy and stress free. Some species (most live bearers) require alkaline water, while others (egg layers) prefer water that is more acidic. Most however, will do well in water with a pH ranging between 6.8 to 7.0. This is best if yours is a community tank, housing a number of different species. If it is not within this range, you might need to make an adjustment.
When buying your new fish, ask what their pH requirements are.
The water in your aquarium can become more acidic when the tank is dirty, especially if there is uneaten food in the tank. Often, all that is required is a cleaning and a partial water change to raise the level to what's more comfortable for your fish. If your tap water doesn't raise the pH level enough, there is a product called "pH Up" that will easily remedy the problem.
Should your water be too alkaline, you'll need to make it more acidic with a product called "pH Down". (This product is an acid and care should be taken when using it, as it will cause burns to the skin.)
When adjusting the pH in your aquarium, you must do so over a period of a few days. This makes the change more gradual, minimizing the risk of too drastic a change, that can harm your fish.