I've seen this thread before...
My suggestion, any of the red slime "cures" which are anti-biotics.
I have never seen a decrease in the capacity of my tank's biological filtration because of using these anti-biotic treatments. *as would be evidenced by the presence of ammonia
Red slime is a hitch-hiker with a simple cure. I spent hundreds of dollars in the past modifying flow, adding certain livestock and so forth trying to cure red slime. I gave up and used one of the antibiotics and, viola, cure.