kribs will eventually get eaten when the ray gets larger. kribs dont grow that big and they swim at the lower levels of the tank. if you do then get lots of caves/flower pots.
Those are made to float holding up a turtle's weight on them. You would need something pretty large to hold one under water, which would displace a large amount of water. Your better off with some tile from the home depot or a flower pot.
You could always do discus I guess. I've heard alot of good things about discus and rays. Thats the only small thing I could think of that wont get eaten.
discus? i really hate them so what about red zebra cichlids they wouldnt be too bad if i had a breeding colony would they cause i doubt a reticulated ray could eat that much...