I've only ever used salt once on one baby clown loach with ich. Salt would raise the TDS significantly, in my case I dosed gradually up until about 700-800ppm, which was the equivalent of the required dose for ich. It took weeks to bring it down. I can't imagine doing the same as a preventative. It cannot be easy on fish at all, especially if one pours the dose all at once and then takes it out all at once...