Probably the best thing to use would be MC bond, which is a little slower than weldon #4. You might try using this for the "sides to ends" joints and then weldon #40 on the bottom, with slightly chamfered edges on the bottom of the sides/ends. But, I gotta say, if you're building a 500 gallon tank out of 1" acrylic, you really ought to know what kind of solvent to use....I mean, you're talking about a very expensive and potentially difficult project. 1" cell cast, Cyro or Spartech, has got to be at least $500/sheet, and I guess you're looking at 4 sheets?

A little off topic, but I'm going to start building a few things again and bought a couple of sheets of Cyro GP, 3/8". I paid almost $200/sheet, which is substantially more than I used to pay. The guy said oil prices had caused the price to rise. Anyone else hear this one? I'm now paying the same sq ft. price for clear Cyro GP that I used to pay for black spartech polycast. I found a place that said they had clear spartech for $200 (5X8) but when I got there it was the same imported crap. GRRRRRR... you sometimes wonder how these guys can hold a job down.