60 ppm nitrates is nothing, in all honesty. rays breed and flourish in amounts exceeding 200ppm in tanks.
it's the nitrites he should be worrying about. 5ppm is no joke to a fish's health. the only thing worse would be if it were ammonia.
it sounds like your tank is constantly recycling itself and can't settle its bacteria colony to support your stock, which is already too heavy for the tank. i'm surprised you can get away with water changes every 4-5 days, it should be more often with that load and those numbers.
do you clean the filters with each water change?
when you clean your filters, do you rinse them in tap water or tank water? tap water will kill off bacteria colonies in your filter media so don't do that.
plants at this point wouldn't really help. can't put a bandaid on a decapitation and call it good to go.
you're right, prime detoxifies nitrites and ammonia, it doesn't magically get rid of them, they are still in the water column till you suck them out with water changes.
however, i personally don't see why you'd need to be using nutrafin cycle as well with every water change. it is a waste of money and is really accomplishing nothing. after 3 years especially, there should be no need to be adding it in all the time. your tank should be cycled.
does your tank have any substrate or is it barebottom? substrate will give bacteria more area to colonize than on the surface of the glass, and next to none in the water column.
remember, for a ray alone, your water turnover rate should be 4-10 times the amount of your tank. in your case, 500-1250 gph turnover on a 125.
that's not including all of your extra stock.