Use a pH tester kit to obtain an accurate reading of the pond pH level. Most tester kits require collecting a water sample in a vial and then adding a drop of tester fluid to the vial. The resulting color can be compared to the tester-kit chart to determine the pH level. Most koi do best in water that is neutral or slightly basic, at a pH level of 7.0 to 8.5. A pH level below 6.0 may be dangerous for fish.
Check the bottom of your pond for debris and decaying organic matter. If your pH level is between 6 and 7, you may be able to raise the pH level by simply removing debris from the bottom of the pond -- the debris may be providing food for pH-lowering bacteria. Use a net to scoop up any visible debris.
Raise the pH level of your pond further by beginning daily water replacement. Test your tap water to ensure that it is at a pH of 7.0 or higher. Each day, remove 10 percent of the pond water with a bucket or rubber-hose siphon, and replace with fresh water. Continue testing the pH level of your pond daily.
Boost a stubbornly low pH level by dissolving baking soda in a 5-gallon bucket, and poring the solution into your pond. Dissolve 16 oz. baking soda for every 500 gallons of pond water. Retest the pH level one hour after adding the baking soda solution.