well, i build it before i checked the reply.... :-
I hope it holds ok.... it is a dual shelf, because I need a way to suspend my light, and i might eventually make a SURGE if flow is not enough, so i can hold the surge tank on top, but i didn't put a cross support on the bottom ot the tank, but i did installed 2 support braces like you on the top (for my light hangers), but i did installed on both levels a 3/4" BIRCH plywood, and also I did painted it today (I will take pics tomorrow) and for the final steep, I also purchased a 1" insulation foam, that i will cut to fruther support the tank, I read that if you install a soft/firm supprt for acrylic tanks is better, since it removes high stress points, if the wood becames uneven..