I have been using a Rena smart heater for about 4 months now and it seems to be safe for the rays (cool to the touch). I have it connected to the input of my Rena XP3 filter. It has worked flawlessly so far. My other heater is in my sump.
Good luck with your rays, they sound to be in good hands.
Treat them to some raw shrimp if you can. Usually you can purchase 50 count bags, farm raised, de-veined/headless shrimp at the grocery store. Silversides are also a great addition.
So very glad to hear that you and rays are well.
Respectfully,
FireMedic.
The good thing is that all my motoros love their shrimp the only thing that I've noticed is they eat earthworms alot more agressively. Yesterday one of my discus was tring to take a bite of some earth worms and my big female jumped on her and started trying to eat her. The discus is in pritty bad shape now. I'll start feeding shrimp again