A simple solution might be to set up a pump and continuously bubble air through the water.
Carbon dioxide in the air can easily bring down the pH of Distilled water (start at pH 7, if allowed to fall any distance into a container and left to stand may go as low as pH 5.6*)
The CO2 bubbled through the pond may help to moderate the pH in your case (just remember that pH is a logrithmic scale)

Good luck.

W


* Experience of filling 100+ litre holding tanks linked directly to a double still