Your problem could be disease, food issues, or the ability of the water to carry oxygen, which can be due to the skimmer being off (it oxygenates) or the treatment chemicals. As a rule, do not add magic cures to your tank. Red Slime Remover is not in itself a bad treatment, but when coupled with low oxygen in the first place (the skimmer off) or poor water conditions what is ok can turn into a problem.
Cyano in itself is harmless, or reasonably so, unless it shrouds a coral and keeps it in the dark. It won't hurt fish. Red Slime Remover I have used---once and long ago; I prefer the 3-day darkness (lights off) once a month and aggressive skimming as a cure for cyano. Consult online for your issues before you let your fish store sell you a magic cure in a bottle. A 90 gallon should give you a lot of leeway, but it can also present math problems when calculating a dose---but you should know fish meds and doses for fish not intended for eating are NOT monitored by the FDA, and most anybody can put something out there and call it a miracle in a bottle. That said, many are good. But some aren't. And if you use more than one at a time it's a potential problem. If you'll take care to get some Salifert tests for the items in my sig line below, and use calcium, dkh buffer and magnesium to adjust your tank to those parameters, that will help both corals and fish. I'm so sorry for the troubles you've had. We've all been there over one issue or another, or it just hasn't happened yet. But your problem is (my best guess) an intersection of no-skimmer with low-oxygenation, most of it produced by the med and the instructions, which are not appropriate for every tank. Red slime is just something new tanks tend to go through, and if you go lights-out once a month for 3 days and skim at your most efficient, that's the best way to beat it.