An arowana would be a perfect tankmate for the tarpons. I had an Indopac that I got in the same shipment back in April that I got my GATF in. By the time the GATF killed it, it went from being the same size as the GATF to almost twice the GATF's size. The GATF got it to jump out of the tank. :/
The moral of that circuitous story is that a tarpon will outgrow an ATF quite quickly. Once my tigers have some more size to them, now that I've figured out how to tame them, I'm going to get more tarpons.
Also, lix, ladyfish aren't quite tarpon. Ladyfish and Tarpons are sister genera in the Elopidae family. Tarpons belong to the genus Megalops, where as ladyfish belong to the genus Elops.