rubbermaid makes livestock troughs. I bought a 50gallon for $70 at a farm supply store. They're sturdy, uv resistant and fairly light. I've got 3 comets, a shubunkin and a sarasa comet in one outside right now. All of them are about 4" long and doing well outside in central NC in december. It's about a foot deep when filled to the brim. There're pics up in my photos.
I got it because I'm not settled on where to dig a pond in my yard yet, but wanted to keep some former classroom comets around after they outgrew the 20gal. My toddler added the fancies after. . .