From what I hear they need super clean, soft and acidic water. They need a long wide tank because they get huge and need their swimming space. You shouldn't have many decorations in their tank because they like to turn stuff over, uproot plants, and pull stuff off the walls. You can't keep them with plecos, or anything else that will hurt their discs, and you can't keep them with fish that will small enough to be considered ray treats. They need to be fed on a daily basis. Did I mention they need amazingly clean water? A good site that has a ray forum is aquaticpredators.com.
I've been building a 6' by 2.5' tank with some extra special filtration. They're definately cool.