Well it's certainly possible to have a saltwater pond. I've seen where people have done coral reef ponds and also shark/ray ponds. Lot's of challenges to overcome though, make sure you do your research.
Yes Here is one at our local shop H20 Exclusives running a custom Nexus 300 unit converted for saltwater. This is actaully the first saltwater conversion kit from EA sold to the USA. It's been working like a champ!