I believe the Lower temp only slows down the beneficial bacteria but doesn't kill it. Outdoor koi ponds get much colder and it doesn't kill the bacteria.
I have had my ray tanks around 70 for periods of time with no problems
I don't think te rays like it and the appetite is not very good but they can tolerate it imo
Never had any problems with any tanks using cold water or gradually changing temps, the only problem I've come across is when drastically dropping water temps.
I was down on 68 few days back, turned the heating cable in the room of and the window was open. Dropped alot during the night. This tank was a pup tank with 4 pups in it, all went ok, but they were not really active at all