Let's do some math here, fellas.
Ocean water is around 32 parts per thousand salt.
Brackish water ranges from about .5 ppt upward to 30 ppt. Freshwater fish can tolerate at least 6 ppt, some can handle more than that. Reproduction of most freshwater fish is affected around 4 ppt and beyond.

A 3/4 acre pond, average depth 6 feet deep, is about 4.5 acre feet of water. 324,000 gallons are an acre foot. So, that 3/4 acre pond is about 1.5 million gallons. 1.5 million gallons, at 8.35 pounds per gallon, is more than 12 million pounds of the liquid. .5 ppt, to make the water brackish, would take 6,000 pounds of salt.

That sounds like ten years worth of salt from the softener, assuming no flushing, assuming none of the animals take up any of it (which they will), to reach that magical "brackish" qualification. By the way, at .5 ppt, you can't taste the salt in the water.

Teach a man to grow fish...
He can teach to catch fish...