I have heard of people keeping freshwater rays in plastic swimming pools or the ones that are not very big but have a liner( like the ones you see Koi in). I have not attempted this but was wondering if anyone has and what they think?
I'd say don't go cheap on your ray setup. Instead of a kiddie pool, go with a polyethylene stock tank. A 400gal 75" diameter stock tank, from gemplers.com will run you about $300 delivered. It's alot sturdier than a kiddie pool. And, you can install a viewing window in lexan or plexiglass to make it a large aquarium. You can even drill holes for standard bulkhead fittings for plumbing your filtration system into it.
my new project is an indoor pool downstairs with water fall... you know like the ones outside but in, and without the pineneedles and bee's. You'de be amazed what pond liner and blocks can do. Just put your mind to it.
Try a pond, but dont go the cheap way, You'd be amazed at what you can do. If you can't have on indoor you can also try an outdoor pond, but buy a portable green house then put the pond in that, it seem's to work pretty good.