I wouldn't mix Oscars and a ray. Oscars are notoriously messy fish and rays need VERY clean water conditions. In fact, I don't recommend rays at all for beginning fishkeepers. If he wants an active and aggressive tank, cichlids would be just the trick. They are also colorful. I prefer small, peaceful tropicals because I personally like to look at a tank to relax. The colors and gentle schooling and movement are good for that. If I had a second tank, cichlids would be fun too though, for a change of pace. Anyway, google up freshwater rays and I think you'll find out theya re tough to keep and since they are expensive, that could just mean you spent a whole lot of money to watch one die.