I made some phone calls to a local acrylic / plastic distributor and was asking about pricing etc. Anyhow, we got to talking and when I told him it was for an aquarium he said it's really important to use "quality north American" brand acrylics, not the "cheap off-shore stuff".
I don't know enough about these things to know if he was giving good advice or just trying to make a bigger sale, but I thought I would pass it along... After all it would really suck to do everything right and still end up with 600 gallons of water in your home...