Checking pH is a simple matter of getting a cheap testing kit, and testing the water.

You can buy one at some of the major discount stores, or a pool supply place for a few bucks. It will be very handy for you.
pH should not be below 6.8, or the water will not support the algae bloom necessary to support the aquatic invertebrates (bugs) needed to help support the fish.
If the pH is not high enough, you will need to use an agent, usually agricultural lime, to get it higher.
How much you will need to use, is dependent on the pH, and the area to be covered. In general, from 1 to 4 tons of agricultural lime is applied per acre. Liming can be difficult work, but it is well worth the effort if needed. It will make every other aspect of your lake or pond management work better. There are some products becoming available which should make lime applications a little less labor intensive.

How often must I lime?

pH should be tested on a regular basis, but re-liming may not be needed each year. One treatment can last for 3 to 5 years. There ares some cases however, where annual re-liming would be desirable. In such cases smaller amounts of lime are needed per application, usually about 25% of the original application.

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Managing Water Clarity