Yeah boy ! With tank that size you don't want any gaps on those supports. I wouldn't go with out a brace on the bottom either if I were you. Your tanks 8' long right? a little under 4 foot on either side of a brace should leave you with plenty of room to work around. Learn to make those clamps your friends dude. You can do some amazing projects with a good saw and good old bar clamps.
Do you have a chop saw or a compound miter? If so cut your legs a little longer then you want them to be on the 1st cut. Then flush up two at a time on one end and flush cut the other end of the two on your miter or chop saw to final length. That should leave you with no gaps at all when you assemble this time.
Pay special attention to your box frames as well you want them flush on the tall side as well as the end. Glue'em, lightly clamp'em, center them up, clamp them down then do the same on the other side. Put your center braces in last on your box frames or its likely to mess you up and take you out of square. Do both ends then place your center braces.
Well I have 3 coats on mine now. One more coat of gloss black then its time for a poly spray. The local Home Depot AND Lowes were both out of black shelving or I would have cut those today.
Ah well .. I still need to make my doors and side panels any way.

I am sure by mid week when I pick up more 1x4's for the doors and side panels they will have them back in stock.
The reason I am using it is the finish on it is tough, and its stronger then 1x4's when laid flat ( I don't want to use 2x4's because 5 2x4's weigh more then 2 press wood shelf planks
