hard to say.. PH swing? wont know if that is happening without a meter. Maybe a parasite? Maybe from slowing starving and just got to unhealthy.
I use this one
http://www.drsfostersmith.com/produ...MI5d3lzaaE1QIVh7bACh36fwsnEAQYASABEgI8m_D_BwE
Since your new to rays you may not notice the obvious signs of parasites. "coughing" "breathing out of one side"...etc.
You got these from jimr...correct? If they are p14 mix, sometimes p14 are the first fish to show signs of a problems.
another thing that is good "insurance" for rays are a quality UV light. Will help with redox, and parasites...etc. Im a believer
My advise, is nightcrawlers. thats where i would start, if it refuses them, and wont get a belly from eating them something is wrong!