In the morning the pH will be low compared to a test done in the late afternoon so it is important that you remain consistent with the times you check your levels. The buffering capacity of the water will play a major roles in the dynamic fluctuation of pH during the course of a day. The pH you record may be unique to your pond. Don’t be influenced by “perfect situations”. It may be slightly alkaline with a pH of around 7.8 – 8.2. It may be that your particular pond has a pH range of 7.2 – 7.6. Or there may be greater fluctuations. This is the natural, daily, pH cycle of your pond. I strongly advise against attempting to adjust the pH to a perceived perfect figure of say 7.2, for the following reasons.
Measuring the pH in the late afternoon you will inevitably find it is on the high (alkaline) side. The addition of acid to pond water to lower the “high” pH can be disastrous at this point in time. The acid will lower the pH and so will the natural pond cycle during the night. The result – a pH crash – with the water becoming several hundred times more acidic in a few hours. As the buffering capacity of the water counteracts the acid a day or two later the pH climbs back to where it was before the addition of acid. All that was achieved was a radical swing in pH as opposed to a gradual natural dynamic change.
Sudden substantial fluctuations in pH values will stress Koi and possibly kill them. Although fish do adapt to changes they do no tolerate sudden, substantial changes in water quality very well. If they do not die, then the stress they suffer may become a contributing factor to lowered resistance and a possible disease infection. Measuring pH should be done in conjunction with measuring total alkalinity. The total alkalinity will give you an indication as to the buffering capacity of the water against pH variations.
The amount of algae (mostly free floating, single cell algae that makes the pond water green) can have a dramatic effect on pH, pushing the pH to very high levels in the evenings after a full days photosynthesis. The opposite effect is observed by a lowering of the pH in the early morning.
The fish are not the only life in the pond. Not only the Koi but the toxicity of the ammonia as well as the bacteria in the filters are affected by pH. The nitrifying bacteria that grow everywhere and in the filters thrive in alkaline water but do not function very well in acidic water. Should your water pH fluctuate through a narrow range, you r water is will buffered. Should there be large fluctuations you may need to buffer the water a little. The pH should not rise much above 8.5 at its maximum reading.
Koi have adapted to survive in a pH range of 6.5 to 9 provided the daily fluctuation within this pH range is not large and providing the pH change is not sudden. Nitrifying bacteria in our filters have an optimum pH range of around 7.8 – 8.5. Their upper limit is 9.4 and around pH 6 – 6.5 the nitrifying bacteria cease to function.