When i got my first ray she was a fussy cow and would only eat live bloodworms so i was buying 500 grams of the stuff from an angling shop every week. Its usually only available in winter but only costs £8 for 1/2 a kilo instead of a quid for about 20 worms in one of those little plastic tubes full of water from the lfs. If i succeed in breeding my rays or buy any small fussy fish i will buy bloodworms this way again.
Dendrobena worms are fine for fish food but an expensive way of doing it, just start a compost heap in your garden (or if you dont have a garden use a family members garden) and within a few months you'll have more worms than you will ever use and be helping the enviroment as well by not sending bio degradable rubbish to landfill.