just keep them. and keep them happy. everyone warns you but each fish/ray is different. i've had my rtc for 4yrs and everyone told me it would be to big in 2yrs. he's still a sweet heart but nowhere near 4ft. or 3ft lol
a pair of rays, in a 180, will be fine for quite a while. they grow quick, splash around, but only you will know when they need an upgrade. buy em, raise, em, keep em happy