If you want tank mates then IMO most would need to keep it warmer , unless they are sub tropical tank mates. I find better healing,eating,growth and less disease in warmer water.Like Matt mentioned Freshwater rays do not need higher temps. Rays live on the bottom where the water is cooler. Nothing wrong with keeping it warmer and nothing wrong with keeping it cooler. It is your preference.
I'd say it depends on species. I have rays that were pulled out of a natural habitat of mid 60s. They definitely seem happier in mid 70s then mid 80s.... lol.
So you reckon mine would benefit from a drop into the high 70's?
I've heard of them being caught in temps of between the mid 60s way up into the 90s, definitely adaptable creatures
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