I used BioGuard (made by ValvTect) on someone else's recommendation. Got it at
Outdoor Superstore for $13 plus $6 shipping (Price increased to $15 now). Put a clean prefilter on, dumped it in my tank. Drove to work the next morning and the prefilter was already clogged with the black stuff. Soon as the tank is low, I'm changing out the tank filter (already had one on hand).