1ppm = 1 mg/l. 40 ppm/mg/l is minimum, I'd recommend anything over 50, but the higher the better. 100 would be nice! Our ponds in this area are naturally around 150+.
Think of the alkalinity in a pond like pH in soil. Too low and the organisms can't utilize the nutrients that are in the water. So if the alkalinity is low, don't fertilize - raise the alkalinity instead. If you fertilize THEN raise the alkalinity, you might have too much of a bloom.
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