I kept 3 small Indo-Pacific (Megalops cyprinoides) tarpons a while back in totally fresh water. Really quick movers. Pretty hardy fish and not that demanding to keep, actually got mine to take pellets.
Wish we get your big version (Atlantic) over here. Ours are alot smaller. I've caught them up to 3 ft long (about 8-10lb) in North Queensland. They absolutely go berserk on a line, jumping and twisting all over the place. Apparently the can get up to 1.5m long, but most in fresh water only grow to 60cm.
Sydney Aquarium has some in fresh that were about 45cm. The common name for them ver here is Oxe-eye herring.
The atlantic version Megalops atlanticus grows alot bigger, read somewhere up to 300lb. The all tackle record was caught in Venezuela at 283 lb. Read an article in 'Sportfishing' magazine about one hooked on fly took over 9 hours to bring it to the boat only to lose it at boatside. Amazing fish-hope one day to make it over to the Florida Keys to catch one on fly