The design looks very good, but one suggestion I would offer is to avoid the use of mdf for the shelves, especially on top of the stand. It's inevitable that at some point during the tank maintenance that you will splash some water on it, and mdf is not water-friendly.
Not a frame comment however, make sure that you create lots of room between the tank cover and the top of the stand. Running out of room there is a real pain in the tush. Design looks nice and clean. Encourages observers to look at the fish rather than your fine carpentry.
Cheers;