yeah a ray would go fine with your RTC's, as long as you have alot of room for them. but you could add them now since you are still building that pond right? ive seen alot of videos of rays that just kinda sit, and then move every once in a while..... but i suppose a tigrinus cat is no different, they are very sluggish. maybe rtc's are the same way, idk. some rays can get a very big diameter to them though, like at least 30+in. id love to have one in my tank with the arowana though. i could do more than one if i wasnt doing a clown knifefish, but yet i need to stick to my roots lol.