A 125G is the minimum tank size for even short term storage of a ray, and that is with only 1 or 2 rays in the tank and nothing else. Most ray species require a tank in the 3-4' by 6-8' size in the long run, if you aren't prepared to get something that size in the next couple years don't get a stingray.
So my advice is don't get a ray in that 75G, if you upgrade down the road think about it again then. As to what an RTC can do to a ray, see this recent thread:http://www.monsterfishkeepers.com/forums/showthread.php?447143-HELP!!!-my-sting-ray-was-attacked
My guess would be the RTC was responsible, but its hard to say for certain.
So my advice is don't get a ray in that 75G, if you upgrade down the road think about it again then. As to what an RTC can do to a ray, see this recent thread:http://www.monsterfishkeepers.com/forums/showthread.php?447143-HELP!!!-my-sting-ray-was-attacked
My guess would be the RTC was responsible, but its hard to say for certain.