It seems very, very rare for an Atlantic ray to survive for more than a few months in fresh water. Plus, with the amount of ammonia output your tank ends up smelling like a urinal, and the ray can't be happy living in that either. From what I've seen, the filtration would be possible for a ray like that if you have the required water volume. Dilution or a very fast drip (squirt?) system would be needed.
To give you an idea of what I'm talking about, a freshwater ray would live comfortably in a 300 gallon tank when it is smaller. The needed water dilution for a converted ray is generally estimated at about x10 water volume, so you would need a 3000 gallon tank to house a small Atlantic ray comfortably. Don't even get me started on how much water you would have to drip to make up for that.
Easy answer? Go salt.
To give you an idea of what I'm talking about, a freshwater ray would live comfortably in a 300 gallon tank when it is smaller. The needed water dilution for a converted ray is generally estimated at about x10 water volume, so you would need a 3000 gallon tank to house a small Atlantic ray comfortably. Don't even get me started on how much water you would have to drip to make up for that.
Easy answer? Go salt.

all i ever see is good deals on large tanks in cali...