Take out 1 gallon of water in the tank, put in a bucket or similar. Pour out half so you only have maybe a half gallon of the saltwater form the tank. Mix in an equal amount of RO
water to get back to the entire amount you originally took out. Pour this back into the aquarium. In other words, what you are doing is simply lowering the salinity of a small amount of the tank water rather than pouring in RO
water directly into the tank.
What I posted is a slower way of doing it but shouldn't stress the inhabitants. As I also indicated, if your salinity is running high, you may need to do this same procedure every day or every other day until it stabilizes.