Wanted to ask if the upper pond flooded grassland or if you graded to clay and created a clay basin. If grassland was flooded and if the organic fraction was very good (as prairie soils often are), this may be the source of nutrients causing your bloom. I mention this because you are not feeding alot and not fertilizing. I'll defer on the aeration question and look forward to those replies.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers