putting a pleco in is a HUGE risk man. they suck the mucus off of the rays and then makes them susceptible to sickness and disease. i'd get rid of the pleco if you want rays. also, do you have sand in the tank? its best to stick with fish that stay on the TOP of the tank so they dont stress out the rays, but if you have some large motoros it usually doesnt matter. but stay away from bottom dwellers like syno cats, plecos, loaches, etc. cuz they'll take the rays food before they can get to it.