Yeah, I photshop every single picture I get off the camera, but they're still no where near the quality you'd see on The AGA International Aquascaping Contest
ACDSee has a great "Auto-Level" feature that will fix up minor color and brightness/contrast imbalances, but nothing this severe.
All of my pictures are without the flash. I tried taking one standing on a chair, aiming down at the tank and it is slightly better, but not great. This is also with my "exposure compensation" (essentially a contrast adjustment or the equivalent of an f-stop) reduced all the way.