I have this gross looking brownish-maroonish slime that's been growing for a while now on my sand. It started out as one little tiny spot. I've been removing it by hand every so often but it always comes back. Now it's on about half my sand. It's even getting on my rocks. I do weekly water changes ONLY with RO/DI. I have no fish in the tank yet. I've also been using PhosGuard. Nothing new in the tank for a couple of months. What is this stuff? What causes it? And how can I take care of the problem. This stuff is disgusting. Any help would be much appreciated.