Maintaining aquarium water pH is one of the most important on-going facets of maintaining a healthy tank. Proper and consistent pH levels are critical to the health and well-being of your fish. Basically speaking, pH measures how alkaline or acidic the water in your tank is. The pH scale ranges from 0 (acidic) to 14 (alkaline); a pH of 7 is neutral. Most aquarists strive for maintaining aquarium water pH at somewhere between 5 and 9, depending on the particular fish.
Keep in mind, when it comes to maintaining aquarium water pH, the size of the tank can make a difference. Larger aquariums are often easier to maintain, especially for beginners. Because the water must be properly treated for the fish and plants to survive, having more water allows you a wider margin of error and a more forgiving environment. It is also important to note that the pH scale is logarithmic. This means that a pH of 9 is ten times more alkaline than a pH of 8.
Maintaining aquarium water pH is best done by testing on a weekly basis. When it comes to making water changes, you can do so monthly or weekly. If you opt for weekly water changes, change 2 to 5% of the water in the aquarium each week. Changing less than this will not generally have enough positive impact to be worthwhile; changing more than this can be detrimental to the aquarium. You should first always note what the pH of your tap water is, after leaving it sitting out overnight in a bowl or bucket. This lets you know what you can expect the water change to do to your efforts at maintaining aquarium water pH.
If you do not perform small water changes weekly, a monthly water change should be done. If done monthly, this change should be around 15% of the water in the aquarium. Changing the water helps remove chemicals that are not filtered out, and helps to keep the gravel clean. This can make maintaining aquarium water pH easier by keeping overall levels consistent in your tank.
If you find that your pH test indicates you may need to adjust the 
pH in your water, there are several ways to go about it. In general, raising the pH level is much easier than lowering it. Low pH can often be caused by “soft” water. You can raise pH and harden the water (making the pH more stable) by adding crushed coral in your filter (depending on the filter type), adding decorations like limestone rock that contain buffering salts, or using specially formulated products to raise pH. To lower pH, you will likely also need to soften the water to prevent the normal buffering properties of the hard water from raising the pH back up. One method of doing this is filtering the water through peat.
Maintaining aquarium water pH keeps the environment stable and healthy for your fish. You should always verify the proper pH range for your fish (generally between 6 and 8, depending on the type of fish). It is also helpful to know the pH of your tap water, to be aware of the effect water changes may have on the pH level.
