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A Guide To pH Levels in your Koi Pond

Controlling Koi Pond pH Levels

The pH level of your koi pond changes depending on what time of day you test the koi pond water. If you test it in the morning, you will get a lower pH reading than you would in the afternoon. Koi ponds tend to naturally have a high pH in late afternoon. The pH level fluctuates over a 24-hour period because of KH (carbon hardness) in your koi pond.

Each koi and koi pond is unique, and pH readings are as well. Readings for your koi pond could fall between 7.1-7.5, 7.6-8.1, or a larger difference between the pH high and low could be present. This is all because of the unique daily pH fluctuations of your koi pond.

This is why you should never believe that your koi pond has to stay below a certain pH reading. Don’t panic if your water reads higher than you think it should! Some koi pond keepers mistakenly believe their pH has to be below a certain level, for example 7.5; when they get a reading that is 8.2, they rush to add an acid to lower the pH.

This is the wrong move!
Adding acid to your koi pond to try to lower the pH will only give your koi a jolt by interfering with the natural pH cycle of the koi pond. Sudden pH changes, particularly a rapid drop in pH, stress out your koi which diminishes their ability to fight off bacterial infections.

Also, adding acid to lower the pH late in the day can trigger an overnight “pH crash” which can kill your koi; you could potentially wake up to a pond full of dead koi. It’s natural for your koi pond to have a significantly lower pH at night; therefore, the addition of any acid that lowers pH will cause the level to drop to a fatally low level.

Your koi have adapted and become used to the natural up-and-down pH cycle, and it does not bother them.

Natural pH fluctuations are caused by fluctuating carbon dioxide (CO2) levels. During the day, plants and algae in your koi pond give off oxygen and use up CO2. The drop in CO2 causes a rise in the pH of your koi pond because of photosynthesis.
During the night, plants and algae use up oxygen and give off CO2; this causes the pH levels to drop.

These natural events are why your pH and oxygen levels are low early in the morning. The koi themselves and the sludge on the bottom of your koi pond also affect oxygen and CO2 levels.