I don't like the way you attached the top and the bottom. It will probably be ok, but you should have dado'd or rabit'd the side supports into at least the top of the stand where the tank sits on. What your design does it put most of the load on the screws and some on the plywood sides you will add.
You will be heavily depending on your plywood for both top tp bottom and diagonal support. You should at least make the diagonal supports so as not to put all your eggs in the plywood basket.
If you dado/rabbit the up and down supports to the frame rails, they will be strong enough for everything and your plywood will be just for added strength.