There is a line of fitting's that are made out of the same material as the pex but are they expensive. It would probably be cheaper to just use PVC.

wall thickness not a problem but 1/2" hooked to a Barracuda might not be the best.

i heard from a buddy of mine, and hes doing up a 140g and he said somthing about the return line should not be smaller then 1".... pex is 3/4"... its your tank but i would go with the pvc pipe even if its a bit more.... the tanks (reef) aint cheap, and not somthing to mess with, and yes.... you can get the fittings out of pex, but you still need to crimp the line with a copper ring... and lets say some how theres a small pin hole in the line/ or not crimped on tight and u dont see it and water passes the copper ring and drips in the sump

and the crimping tool can run you $100.00+... and the plastic fittings have blowen off befor... cheap ways aint the best...

Thanks guys I am going to go with PVC