Is this red stuff spreading to cover your sand bed as well? Is it slimy looking? Does it come off in sheets if you pick it off? If so, it is probably cyanobacteria.
I had a bloom of cyanobacteria in my tank that always had bubbles in it. The thing about this is that it is a bacteria, not an algae, so the standard treatment of limiting nutritients and light doesn't make it go away. You need an antibiotic to make it go away. I used Red Slime Remover, and it worked like a charm.
Also, cyanobacteria is ubiquitous in the environment; it actually helps prevent soal erosion in the high deserts of the West. I am certain I introduced it into my tank after I had been working in my garden. I hadn't used gloves, and hadn't cleaned under my nails to make sure all dirt/soil was gone. Now I take much better care to ensure my hands are clean, and I have a pair of gloves on order.