10 gallons is an awful small tank to QT a rabbitfish in, though 75 would be a bit overkill...lol. I have an empty 29 you could use. The problem now, is that all the fish in your tank have the potential to get Ich. To properly treat for Ich, you need to get ALL fish out of your display and QT them, while keeping your display fishless (No Fish) for approximately 8 weeks. During this time, all fish in QT should be treated with hyposalinity. In this 8 weeks, any Ich in the display will die from lack of fish.
Unfortunately, the only way to assure no Ich in the display is to keep it fishless for this entire 8 week period.
As for hyposalinity, you should invest in a refractometer. They're much more accurate than a hydrometer and you'll need to keep your salinity at 1.009 in the QT.
You'll start your QT at regular salinity, matching your display, and lower it over a period of about a week to 1.009. It'll need to remain at this level for about 6 weeks and at least 4 weeks after all visible signs of Ich are gone. Then you'll slowly raise it back up, to match your display, over a period of about a week. Once it's down to 1.009, it's important that it stays that low, so regular, daily topping off and testing of salinity will be needed. This will keep evaporation to a minimum, which will effect salinity levels.