What kind of filter and media are you using? I had a planaria outbreak a couple of years ago and I had gravel substrate with a large sump full of pot scrubbies. I ended up going bare bottom and stirring the pot scrubbies every week. I would even take them out and squeeze them out. It took months before they went away. You can always put some bala sharks in the tank as they will eat them or even clown loaches. I would assume your ray is still small so just get ones large enough the ray can't eat them.