They can live in salt or freshwater. They do grow slow but the Florida state record for Tarpon is 243lbs so.....maybe a bigger tank?

Average for a fisherman is 40-50lbs.
The Indo-Pacific Tarpon can reach 60" and weigh between 32 and 40 pounds.
The Indo-Pacific Tarpon (Megalops cyprinoides) is a little smaller than the Florida Tarpon (Megalops atlanticus)