Read up on it- 3day black-out period. Most reefer do this once a month. Plenty of good information out there on it .Remenber that if you dont remove the causes you cant completely cure the problem. So this is only a band aid so you wont see the red slime and start to find the source of your problem.
When I did the 3day lights out. On the last day I did a 10g PWC
on the cube & the next day another 5gal. I had a few things going on as this bio-cube was a friends and I started to look for problems as to why. This is the reason I like to start everything myself and not get someone else aquarium.
Didn't know the age of the lights so I changed that first. I cleaned the back chamber cause it was full of junk and the sponge was clogged & what might have been the source of the problem. I rearranged the rocks & added a 240gph powerhead & changed the stock power to a stronger pump 400gph (might be way to strong but its working.) I cut back on the feeding as well.
I did plenty of change so cant pin point what was the cause of the problem, but it took some time and before I knew the red slime that was growing in one section of the sand bed and the rocks were gone.