Generally, I would double the boards going both around the perimeter and across the middle. If those are 2X4s I wouldn't trust them to support the entire weight of the tank as single boards. You used doubled boards for the posts, why not the beams.
Skinning in plywood only helps if you use a lot of screws around the perimeter. You may want to go looking at some place like HomeDepot or Lowes for screw in metal corner brackets. These will help resist racking of the supports and help keep things true.
Another thing you need to consider with a stand design, t's not just about supporting the weight of the tank; its also about keeping a flat top for the tank to sit on. If the stand is not strong enough, it will flex causing the top to bow. If its an acrylic tank, the tank may want to bow outward. If its glass, you will be stressing the bottom and sides as they try to resist the weight of water. In either case the more the stand flexes, the more stress on the seams and the higher the chance of a leak or failure. Overbuilding the stand will minimize flexing of the stand top and better support the tank.
