The key to successful treatment of ich is to make sure that if the tomites are free swimming at a particular moment, they die before they get a chance to set up housekeeping on a new fish. The specific gravity is not something that I have ever tried to measure, so I follow the dosing level in that link in my signature. The teaspoons per gallon measurement is plenty good enough to wipe out ich. The reason you need to get serious about the SG of your water is not that you couldn't get it right with a simple measurement. In a brackish tank you don't want it varying a lot due to things like evaporation so you actually need to measure it. In this case we are dealing with a one time dose to kill off a parasite. The long term effects of evaporation and similar things just don't play into it.
