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Why Is My Pond Alkaline?

| Updated September 26, 2017

Outdoor ponds are a great way to add color and life to your flowerbeds and garden areas. Water quality is not as important in strictly plant ponds, but if you plan to add fish to your outdoor pond, keeping an eye on the quality of your water will go a long way in helping you keep your fish healthy and active. Pond water is generally neutral when it comes to pH, however in some instances, it can become alkaline. Determining the cause of the water alkalinity is key to correcting the situation.

Proper Water pH

Pond water should be either neutral (7.0 pH) or slightly alkaline (up to 9.0 pH) in order to be healthy for your pond fish. Testing the water regularly will help you maintain the proper pH for your pond. If your pond water exceeds 9.0 on the pH scale, you risk endangering the health of your fish, and they could die. pH measures the amount of acid-forming free hydrogen ions that are available in your pond water. The more ions available, the more acidic the water.

Algae Control

Photosynthesis is a process whereby plants, usually algae, use the sun’s energy to convert carbon molecules into sugar molecules that it uses for food. Dissolved CO2 in the water causes the water to be slightly acidic; however, when photosynthesis occurs, the amount of dissolved CO2 drops, allowing the pH to rise, which translates into more alkaline water conditions. If there is too much algae in the pond undergoing photosynthesis, the water can become too alkaline. To prevent this, keep all forms of algae under control.

Sludge Control

Another cause of alkalinity in ponds is an overabundance of sludge in the bottom of the pond. This sludge is a combination of fish waste and other organic material that utilizes oxygen in order to break down. The process releases large amounts of dissolved CO2 into the water, which creates acidic water. However, the presence of large amounts of dissolved CO2 boosts algae formation and increases photosynthesis, which pushes the pH level of your pond up. Regularly cleaning the sludge out of the bottom of your pond will help stabilize the pH of your pond water.


High pH levels can also be caused by acid rain and soil runoff. These conditions can be remedied by adding a pH-dropping chemical to the water or by preventing the introduction of rain runoff into the pond.

Testing Frequency

Test your pond water at least once per week and adjust it if it is too acidic or too alkaline. It is best to test your water late morning in order to obtain the most accurate reading, since photosynthesis can artificially raise pH levels in the late day hours.