i'd remove either the ray, or all of the other fish.
ick treatments are poison to rays, i wouldn't put a drop of that stuff in my ray tank.
you could also dissolve salt into a bucket of water, then pour the bucket into the tank. (make sure the salt is dissolved into the bucket water before pouring in.)
then turn up the temp in your tank higher, can kill off the ick. make sure you have an air pump going as higher temp and salt in the water make for less O2.