To ensure an accurate measure of your pool's pH, test the water under usual conditions, some time in the afternoon. Use one of the many pH testing kits available on the market --- many of these are tailored for testing swimming pools, and contain useful, color-coded guides to help you. Take sample water from about 12 to 16 inches below the water line. Use caution and carefully follow the instructions; using the improper volume of water sample can lead to a false test. Test strips --- pieces of colored paper, known as litmus paper --- are cheap and easy but not recommended.
Adjusting pH Levels
In general, increase pH by adding a base to your pool water, decrease pH by adding an acid. If your pH level is below 7.2, add soda ash (sodium carbonate) to increase your pool's alkalinity. If your pH level is above 7.6, add muriatic acid to increase your pool's acidity. Whether increasing or decreasing the pH level, add a small amount first, and then retest the pH level after five to six hours of constant filtration.
To add soda ash, first mix the soda ash in a bucket of pool water and then pour the solution into the pool. Pour slowly; adding all of soda ash at once will cloud the water. To add muriatic acid, mix the acid to 5 to 10 parts water and pour into the deeper parts of the pool. It is important that the pool water is circulating when adding either the soda ash or muriatic acid, especially since muriatic acid will otherwise settle to the bottom of the pool and etch the plaster.
Maintaining Proper pH Levels
Invest in a pool maintenance guide. Test your pool water regularly, somewhere between once a day and once a week. Your pool's pH levels tend to rise because of human waste, particularly sweat.