Are the corners 2 x 4's? If so you are probably ok removing the center brace as long as a piece of dimensional lumber runs across the top and bottom.
A pic might be helpful though...
10g FW: 1 Dwarf Puffer
46g FW: 3 Oranda Goldfish
10g FW: 3 cherry barbs, 6 Red-Eye Tetra, 1 Glo-Fish