Hi Matt,
A few years back, the LFS decided to bring in some teacup stingrays, but all five we're refusing to eat. So, the manager had me take one home to see if I could maybe do something. Well, the little bugger just refused to eat and died. They had me bring home another one, and although I managed to get this one to eat, he just wasn't eating enough, and eventually died. I was feeding ghost shrimp, as the rays were very small, only about two inches in diameter, and even tried chopping up some earthworms. The second ray preferred the earthworms.
Anyways, from what I know, the larger rays are hardier and easier to keep. I can't tell what type of sand you have in your tank from the photo, but make sure you are not using silica sand, as it will cause scratches in your rays skin and leave it open to infections.
Here are some of the basics in keeping motoro rays:
Water temp 75-80 F
pH 5.0-6.0 (although I have read you can keep them in pH as high as 6.5)
As for softness, there is quite a bit of discrepency in the information I have found. Some books say they prefer softer water, but I have also read that they can be in water as hard as 10 dH. Let me check the master index for their natural habitat, yep, says 10 dH. You might want to get another opinion on this.
Rays prefer to eat live foods and should be fed 2-3 times a day. Blackworms, tubiflex worms, ghost shrimp or other shrimp (has lower nutritional value, and should be fed as a treat), earthworms (should be dead first), feeder goldfish, crickets (also must be killed first), small chunks of fresh fish. They key is to provide a varied diet. The cool thing is that rays can become quite docile and tame and will learn to be hand fed.
Hope this information helps, double check on the hardness though.
Mogo