I think I know what happened. The fertilizer you used has high
phosphate. The Turf Builder I used has a 15:1 N:P ratio. There is a
lawn fertilizer by Scotts called Green Sweep that has no phosphate. In
fact it has nitrate, potassium and iron. That is about as good as it
gets if you don't have to worry about killing your fish. I was not
able to find Green Sweep in my local store. That was why I went with
Turf Builder.