Tropical Fish Pond on Woodward just south of 14 mile. I wouldn't go just for the used stuff though unless you live with in a mile or two of it like I do. Because they don't have very much used stuff at all. Mainly just a shelf in the back with a few heaters, lights, air pumps bunch of 20g tanks and assorted other common everyday items. Nothing too special but I do like to stop in from time to time just to see what sort of used stuff they have.
I did see that they do have a note on the wall that one of the employees are selling a metal Halide light set up for $100 though when I was in there the other day. Seems like a good deal on that, but no idea of the size or condition of it but thought that was worth noting.