discus in an aro tanks is asking for disaster. a beautiful expensive arowana like that shouldn't go to waste.make sure you get a cert and check for the micro chip. as for the ray if it's high enough and you know the dangers of having both in the tank go for it.the aro will probe things in the tank for fun so if it does the stingray and get stung... well... congrats you'll be on another waiting list lol