What Are the Causes of High Ph Levels in Ponds?

Image by Flickr.com, courtesy of Brandon Fick

Various sources in a pond can cause high pH levels. Pond fish prefer a pH at 7 or slightly, and the pH must remain stable for fish to stay healthy. A pH above 9 can kill fish. Identifying the cause of high pH quickly can save their lives.


The pH balance of water refers to the measurement of acidity or alkalinity in a pond. The pH scale has 14 numbers with 7 at the middle of the scale as neutral. The lower the number, the more acidic the water. The higher the number, the more alkaline. A pH above 9 is usually caused by a pollutant.

Alkaline Water Source

Water utilities treat tap water with calcium hydroxide, making the pH slightly higher than 7. This prevents acidic water from corroding pipes by making the water alkaline.

Buffering System

A buffering system like limestone rock, as well as limestone chips in the pond filter, can cause alkaline water, or a pH above 7. However, a buffering system will keep the pH stable and below dangerously high levels.

Lack of Carbon Dioxide

Excessive algae can consume high levels of carbon dioxide in the pond water and raise the pH, sometimes dangerously high.

Causes of Excessively High pH

Pollutants that send the pH to 9 and above often are exposed cement work or builder's lime.