I've figured out that the brown fluff is mulm! I'll collect it and save it when I setup the tank with new substrate. I plan on doing a layer of peat with mulm, then my current MTS, then Flourite. How does that sound?
I see your point Tom. I'll try the Flourite black sand. I'm going to remove all fish and plants and pretend like I'm setting up a new tank.
Also, is Flourite black sand like the grain size of PFS, or is it more like gravel. Would it be suitable for pygmy cories?