after 2 years my retics continue to only eat live blackworms and nightcrawler pieces ( my female does eat the occasional goldfish as well).. the tank gets massivore as well as chunks of fish (everything from smelt, to salmon) California blackworms iir also offer blackworms frozen as well as live as an option... and like said tank-mates depend heavily of the fish themselves.. personally I keep gar and some young catfish w/ mine but also had great success keeping spiney eels to big to eat, imo the biggest thing to be careful of is to not over-load the tank as rays produce heavier then average bio-loads so "stock lightly". I'm lucky enough my lfs orders in blackworms so I buy directly from them opposed to directly from CBW. 1/4lb last my 10" and 8" rays approx. 24-36hours if I don't feed nightcrawlers for a day or 2 prior or up to a week if I feed them nightcrawlers every night. Or they will eat up to 6 nightcrawlers a day ( good sized ones I cut into 1/3's or 1/4's w/ their small mouthes I ere on going with a lot of smaller pieces vs bigger ones. yes I have a dedicated pair of scissors for cutting them...)
Live blackworms imo are great for bulking up new rays/finicky ones since they will live for weeks in a tanks substrate.. I find them in my filters as well fat and sassy. a "Must" have for tiny WC rays.
If you don't mind me asking where in WI are you? we have a lot of lfs that carry blackworms.