From my chemical back ground you could get away with some Phosphates. Your Bicarbonate will buffer the PH keeping it around 7.5 after you add some phosphate. This is what I understand will generate your phytoplankton bloom.

Dropping the PH a bit with phosphates because they are low, will free up some calcium, Magnesium, and potassium. When the PH is higher than 7.5 these are present but mostly tied up with a strong electron bond.

If some one would chime in with the perfect pond water I could calculate the % of fert to bring you in to that perfect balance. It may not be that easy. These numbers may change or fluctuate over the course of the day. I know the PH will I have seen it.

Cheers Don.

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7/8th of an acre, Perch only pond, Ontario, Canada.