Right now, there aren't any fish in Tartarus in the river after the first battle. I'm sure Charon wouldn't mind if we fished from there. Here's a few fish suggestions:
Hades Doodlefish (death, common) Petrified Fish (myth, common) - A fish that has been turned to stone by Stheno Limbo Koi (balance, rare) - A nearly flat (horizontally) fish wearing the same kind of hat as the Face Palm Cerberacuda (fire, epic) - A sleeping fish with three ghostly fish heads circling above it Obol Eel (storm, sentinel) - An eel with a big bulge in the middle from eating a silver coin
Also, the Poseidon Doodlefish (ice) could be out by Atlantea with the other fish
While I love the idea of adding fishing to other areas I think this would be a bad area for it.
It's level locked. No-one below level 90 can enter, even by porting. There would be all kinds of issues if any exclusive fish were there and then came up on fishing tournaments or fish of the day etc. It's bad enough there are 2 exclusive fish only available if you happen to have friends who own the Fantastic Voyage Expedition and Private Fishing Retreat.
Also, there are already issues in Tartarus with people teaming up wanting just Cronus V full dungeon. We would also have to add the further complication of people only wanting the first battle so they can go fishing.
I think keeping this location from further complications as far as battling is concerned is really important. I would also be very disappointed if fishing became level locked, even if it was made impossible for any exclusive fish to come up on tournaments etc.