A couple of LFS have SW ponds. Some are concrete block construction with epoxy liners, other EPDM. Filtration is unknown because it is out of sight, which makes it that much more nice, IMO.
One is a look down reef probably 10-12' x 4', MH pendants, full of various livestock including a breeding colony of Banjji Cardinals and monster leathers.
The other is a Nurse shark, figure 8 style pond. Filtration that is visible on that one, a huge upper/gravity return sump with mangroves and a ton of LR. Both have been running for years and seem to be healthy systems, but I have never really asked and tech. questions about them.
Another LFS has wood framed ponds with EPDM liners. Again, hidden filtration and equipment. One pond has smaller black tip sharks and a few small tarpon in about a 12x15x2.5' pond. The other pond is a small bat and Atlantic ray pond, very cool since in is pretty shallow and the rays are very curious. Filtration looks to be a sump of sorts with a very large return in the form of a waterfall about 16" wide and a foot drop from a large resivor placed in a corner of above tank.