Given the quantity of water needed, I am reluctant to suggest anything. The only beneficial way to soften the water is by distillation ... any other means will just be a replacement of one ion for another and that may be more harmful to the pond life than untreated water. The cure you need is a deluge to flush the pond. 3000 gallons a day is 0.221 inches a day in 1/2 acre. That may very well be the evaporation rate so if the level is steady in a 1/2 acre pond with that influx ... the pond appears to have very low filtration. You need a gully washer to flush things out.

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