The biggest problem is with it being in freshwater, it will prduce alot of ammonia to combat not being in saltwater.
With your pond and filter, you will help lower the ammonia but I would be really careful, test water alot and prepare to do large amounts of water changes.
Even if the ammonia amount is low, it still will affect the ray over time.
there was a posting on MKF about a guy having a Atlantic ray in a 100 gallon tank and it started to curl no long after he got it. I dont remember much about posting, and I know you have way more water and filtration than he did.. i would just still keep on eye out..
I would if it was me, just get a fully freshwater ray or rays...
but bing in Florida, you may have to work on that a bit, get permits..