nayr, I am not using any sort of fertilizer, and it is only (temporarily) stocked with 20 or so feeder Danios. I was under the impression that soil produced large amounts of nitrogen.
baumer, I am a more than a bit confused. By photosynthetic, I take it that BGA can't grow without light, but it's under the sand, even in the middle of the tank. (The BGA in the second pic is only on the surface because I stirred the sand a bit.) Would "removing as much as I can" involve changing substrate? And would killing it with H2O2 require emptying the tank?