What if I took 2 2x4 and braced the back ?? It will be screwing or nailing into the Stand support boards mabey a 3rd piece between the 2x4s under the tank

2x4's are only actually 1.5", so you'd need a bit more of a buildup in there, but you could build a quick frame in the back and attach it to you stand. as long as it's nice and flush at the top it should work fine. if you build it like a door header you will have lots of support and should only cost $20 tops. I assume your tank is 48" long?

It's 24 a 20g

Remeasured it and it's just under 1.5" so 2x4 should work and when building a header is the support 2x4 between the top and bottom boards or they from the floor to the tank

With a header you have 2 2x4's on edge, and fasten them to uprights that are floor to tank. Here's a pic of what i mean: You wouldn't need the cripple or top plate, just the header, jack studs and king studs.

http://t2.gstatic.com/images?q=tbn:A...byceojjOCbZIuF


If you only are going to go one wide set it up like this, except with a bottom plate and another support in the center if you can fit it:
http://t2.gstatic.com/images?q=tbn:A...5OVkFqHHpb3gNT