The1and only;3918703; said:Weld on 16 is stronger than 4 but is much thicker to work with. 16 seems to not dry as clear. If it is on the bottom of the tank an you don't care what it looks like I would use 16. You can get it at a place that sells acrylic or some where that makes custom tanks.
I would use google and start calling around. It should not be hard to find seeing as you are close to Chicago.