Mike I had asked someone to do the math in the first post. Math is not my strong suit. Natural seawater is about 35ppt (parts per thousand) IIRC .03% is 3ppt. So will a math wiz please tell us how much feshwater you would need to take out and replace with natural seawater at 35ppt to raise a 20,000-gallon pond to .03%? If you had a decent refractometer it shouldn't be much of a problem to tweak it. Even adding water softener salt can get expensive if you have a really large pond, especially if you live on a tropical island.
Mitch