Prime protects your fish as the tank is cycling from the effects of toxic levels of ammonia and nitrite.
I know some folks in saltwater use RO, but it is for different reasons, not related to the cycle process, and I will leave that to them to explain.
HOWEVER it sounds like your nitrite level is getting the best of your fish, and it sounds like you need to do two things.
#1 about a 50%
water change.
#2 use Bio-Spira for saltwater to get the bacteria jump started so you get through your cycle without any more fish problems.
You should NEVER add fish before the cycle is complete. People do this all the time, and it is hard enough in freshwater, but
saltwater fish are so expensive and even more sensitive....
The Prime should have helped, but not knowing how much you were using etc, I just can't say what went wrong. Good Luck