Any ideas on what you want to do with it? With 18W over a 7G, I'm not sure what type of corals--if you wanted to keep any--would work under it. Some zoanthids may work, but I bet they would stretch for the light and not grow too well. Mushrooms may be a better bet as some of them live in dim light.
What are your plans with the Fluval filter? A powerhead may be a better alternative for water movement, but I have no experience with cubes. Powerheads usually provide more bang for your buck in the water movement department. Personally I use the Maxijet brand of powerheads.
You might be able to try a single fish or a shrimp or two in there. A small one like a goby might work.
How long has it been set up again? If since May with Liverock, any cycling should be ready for a fish or a couple of corals. It's the light I'd be more concerned with at this point.