IMO 3/8" minimum is what it *should* be, assuming you are using a perimeter flange and a crossbrace. Should end up with two openings roughly 18 x 12".
There are a coupla ways to find a good thickness, one is the Cyro formula, which has it's flaws, but is still the best freebie thickness calculator. An Excel spreadsheet is available at
http://www.regalplastics.net/aquarium.htm
Another way is just a guide:
1/4" for up to 12"H x 96"L, more than either - move up one thickness
3/8" for up to 20"H x 96"L, more than either - move up one thickness
1/2" for up to 24"H x 96"L, more than either - move up one thickness
3/4" for up to 30"H x 96"L, more than either - move up one thickness
1" for up to 36"H x 96"L, more than either - move up one thickness
This assumes a 3" perimeter flange (aka eurobrace) and 6" crossbrace for every 24" of length.
Keep in mind that these are *minimum* thicknesses, if you can go thicker - please do
HTH,
James
