1) Remove his tank-mates.. ALL of them
2) Get live blackworms.. for a 125 approx. 1/4lb... rinse them very well then spread them around the tank ( turn your filters off for about 15mins until the worms burrow down into the sand) You will need to do this 1x a week at first and as it picks up it appetite more often the goal it to keep food constantly available at least until it gains sufficient weight to start trying/adding other foods. Frozen bloodworms are not nutrient dense enough to help your ray ( they will slowly starve even while eating massive amounts)
3) get yourself a bottle of Prazi-pro ( you will need this once the ray has put on some weight and has acclimated to captivity. imo any and all wc rays should go threw a de-worming/ip process, they are silent killers and when used properly prazi is about as safe as salt.)
4) Do 25% WCS every 3 days ( doesn't matter your perameters the freah water is whats key, don't syphon your sand. Just remove from the top/mid level)
These are the basic steps I followed when I bought all 3 of my retics home ( I recently lost my smallest to a nasty columnaris outbreak unfortunately but the other 2 pulled threw and are thriving once again, though i'm pretty sure my female absorbed what would have been her first litter =/) That was over 2yrs ago now...
Be prepared for a larger tank... your ray even if a male will eventually need at least a 180 6'x2' footprint. And even larger if you have a female.
Retics can and are hearty ime once acclimated and their basic needs are met.