I know I could figure it out by playing with it, but I'm afraid to hurt my fish in the process. So, to make 50% water changes in a brackish 10-gallon tank, how much marine salt do I add to the new water (5 gallons) to keep a SG of 1.010? The box (Instant Ocean) only gives measurements for full SW tanks. Would it hurt my puffer if there are fluctuations? Thanks!