Heavy red slime outbreaks seem to be associated with new systems. I certainly had it within my first few months of running my tank.This was maybe 13-14 years ago. I did constant water changes, removed everything I could. This was before I had converted my tank to a reef, so I was only using 2 NO 40wt fluorescents, one of which was actinic. It was also before I knew that filter pads added nitrate. At that time I was running a power filter and a skilter. (Yes, that's bad...but it did skim...was certainly no worse than a sea clone and way better than a prizm.)
When nothing worked, I bit the bullet and used that red slime remover. It's erythromycin powder, an antibiotic. I have to say that it did work....within 3 days the cyano was gone, and its remains cleaned out, never to return. No damage was done to the tank or its inhabitants. Still, it was an act of desperation, one that should not be done except under the most desperate of circumstances..and at that time, I did not know that the product was an antibiotic...found that out soon after.