Gotta agree with duanes here, i’ve treated dozens of fish with ich and have never lost any when just using salt at 1 tablespoon per 2 gallons. In my opinion the issues with ich happen when too many water parameters are changed at once and on top of dealing with ich, the fish is dealing with osmosis from the salt, adjusting to medication and now fluctuations with temp, that’ll kill it faster than the ich. And now if the dose of salt isn’t high enough to kill the free swimming trophants, the increased temps can rapidly increase the ich life cycle. Most of the time, I find people don’t put in enough salt to do damage, i’ve gone as high as 1 tablespoon per 1 gallon for scaled hardy fish like oscars, pacu, and they handle it perfectly fine, i’ve even put scaleless fish like rtc through 1 tablespoon per 2 gallons and he’s now 2 feet long. You can look up whatever fish you’re treating and put brackish behind it and if it inhabits any brackish waters naturally or have adapted invasively to brackish waters in florida and ppl fish for them there (peacock bass, oscars, etc), then you can assure they’ll tolerate 1-2 tablespoon per gallon which is more than enough to treat ich, and whatever secondary bacteria infections come along with it. If you wanna ne super accurate you can weigh it out and measure ppt like duanes said, but 1 tbsp per 2 gallons works perfectly fine as well. Hope your oscar is doing better, they’re usually super hardy and can handle close to brackish conditions.