1st of all the powder blue is so prone to cryptocaryon that it ain't even funny. The powder blue is for experts only. Also you have too many fish in your tank (1 inch of fish per 5 gallon max and I stay at 10 gallon just to stay safe). Your tank has not been set up long enough to carry that size bioload. You are overfeeding and that is why your nitrates are so high. Any measurable nitrite or ammonia is not within normal range. Nitrite is a killer and when coupled with high nitrates is super deadly.

If you are not adding anything to your tank to raise your pH, alk, or calcium then you need to check you pH test kit. Also NEVER, NEVER, NEVER medicate your display tank. Especially with a bacteria killer such as furan. This is another reason why you fish are dieing. There is now no bacteria in the tank to break down the ammonia. Take your fish to a friends QT tank or set yourself up one. The only problem is that with the size of your groupers you will need another 125 to do this. You should treat ick (cryptocaryon) with hypo-salanity, not meds.

As for the salt, I don't necessarily agree that Oceanic is for coral systems. If a salt is good for corals then it is great for fish as well. But never the less, you need to start using RO/DI water and a quality salt. IO is okay and I will not go into a salt debate....I use a different brand and have very stable pH, calcium, and alk.