[Myka]

I had something similar like this happen in a client's tank. Everything was a rusty brown - the skimmer was overflowing rusty brown foam, the filter pads were rusty brown, there was a dusting of it on the rocks. Couldn't see in the tank. It did this overnight. I stopped by, did a waterchange, blew the rocks off, cleaned the skimmer out, replaced the carbon, and replaced the filter pad. The next day it was cleared up. The only thing that makes any sense to me is bacteria. That's the only thing that really comes and goes that fast besides something spawning, but I don't know of anything that spawns rusty brown.