i was thinking about doing a Polluted theme. leave some hosing in the bottom, maybe a jar, 6-pack rings, find some other aquarium-safe trash. set it in the sun for a bit to give the junk a nice coating of algae. put some driftwood in there that will leach tannins and darken the water to give it a dirty look. a school of some sort of catfish would go well in there because they're associated with eating trash... or crays
You could get some nice decorations, drybrush weather them, and then put aquarium safe sealant over them. It'd look dirty.