I have used black before (makes my heaters and any cords in the tank appear invisible) and, to be different, I used some thin cork sheets on one tank. I really liked how the cork looked (it was a freshwater tank), but the black seems to really make the other colors pop. If you really want to try some different looks, check out the specialty paper for scrapbooking in Michaels or another hobby store. You can get almost any look you want. I mounted paper to craft foam and then attached it to the tank. I don't know how this would work on big tanks, though. I have used it on a 55 gallon and smaller.
PS - How are ya? I still haven't gotten the aqualight you sold me out of the box. I moved shortly after I bought it, and I am still moving in. It's always something.*
Rosanne Rosannadanna, approx. 1977