Rob from www.Altisreef.com uses them. They are definitely sturdy. I believe you can find them at farm supply stores. They are often used as water containers for farm animals.
They are very tough. the one in the pic looks like the 300 gal size. i have 2 of those, 8 5o gals and 4 100 gal. You can get them from agway or tractor supply co. the 50's cost about 65-75 bucks. the 100 gal costs about 85 bucks and the 300 gal is about 250 bucks.