Keep in mind the fact that the "freshwater" rays in the St Johns derive from marine populations. Yes, they can tolerate fresh hard alkaline water, but at a large price. To maintain proper osmotic balance, the rays put out TEN TIMES the amount of urea that their marine siblings do. To be able to flush their systems of this much waste, they also must take in huge amounts of water. The water that their systems take in has to be pristine with zero ammonia, nitrates, or nitrites. This is a species that requires over filtration and regular frequent water changes to maintain its physiological balances in an aquarium environment.