you got a good point... keeping Stingrays is a challenge, not because it's quite difficult (depending on species), but just because we're talking about BIG fish, which can live very long and are quite demanding considering food and water and space... which means : COSTS !
As you are in a "thinking phase", I recommend you 3 books (you'll find them on Amazon)
1. Freshwater stingrays (an in-depth survey of these magnificent fishes) by Hans Gonella & Dr Herber Axelrod (ISBN 1-84286-083-6)
2. Freshwater stingrays (a complet pet owner's manual) by Richard Moss (ISBN 0-7641-0897-2)
3. Aqualog special Freshwater stingray from south America by Richard Moss (ISBN 3-931702-87-1)
What you should consider first is which ray you want to keep (and what other fish you want to associate with, if so), and then offer them the biggest surface as possible... don't underestimate the efficiency and power of your filtration. Don't underestimate the amount of food they'll need... don't underestimate that in your country they may be a few trainings & regulations you must follow as we're talking about venomous fish.
I also suggest a few websites... IMO, the best stingray keepers are in Germany (or in the Nederlands), and that for, it's useful to read-understand German... But I'm quite sure you'll find good websites in English too...
a few tips :
1. read
2. ask
3. think
and then, maybe, you can begin to plan a giga-tank for stingrays...
Oh, and I forgot, of course, the validation from your Department of Interior
