Don't know what kind of acrylic you're using (cast/extruded) but one would have to be very negligent to "bump" 1/4" acrylic for it to crack - unless you are bumping it with a hammer & chisel. 1/4" acrylic is pretty strong under the described use by Doug; it will scratch if bumped with a rock but very unlikely it will crack. Doug I would use cast acrylic if you are planning to do this - much stronger.
Also, for 600 i think you can get a tank from Mitch at Interamerican
http://go.to/interamerican with regular glass. I just bought a starphire 180 g with exterior overflow, polished edges and eurobracing for 1000. Shipping is $100 + taxes of $77 total $1177. I think I would have paid around 800 total if it wasn't for the euro bracing/polishing and starphire. I think you can get basic 120g for about 600-700. Give Mitch a call, good guy to talk to. (shipping has to be to a business though)