Around here I can get this blue craft foam thats about the stiffness of your typical styrofoam but stands up to carving much better. A friend of mine makes modles and things and he uses the foam to make landscapes similar to the way you made the 29 gallon one (cutting several thiner pieces in a rough similar pattern then sculpting it after there all glued together).

What I'm wanting to know is what sort of sealents can be used? The spraypaints and stuff my friend uses are obviously not frog friendly but I dont know what would be safe.

As for the shaping, I have no real idea yet. I thought about making smaller peices and glueing them with aquarium grade "glue" (I cant think of the right word ^^; ) to the back and sides but now I dont know. I think I'm gona wait till I get the actual 18x18x24 tank and just stair at it for a while, see what it wants. I know. That sounds really really REALLY dumb but thats how I think of art when working with a new medium. I look at the medium and thing "what does it want to be?" and just go with what my instincts tell me.