The new tank is going to be 30" long by 12" wide by 12" tall so I am designing it to be 30 1/4" long by 12 1/4" wide so there is a little wiggle room to get the tank in and out. I am making the basic frame out of 2x4's and then I am going to cover it with 3/4" pine. The 2x4's are going to go together with half lap joints and the pine will be glued and nailed onto the 2x4 frame. I believe that I should be able to make it out of 4 - 2x4x8's, 3 - 1x8x6's, 5 - 1x6x5's and half a sheet of 1/2" plywood.
Here is what I came up with in Sketchup.