What do you do with fish phantom hourglass?

Answer 1

After you catch a Skippyjack, go back to the
Old Wayfarer and then go catch a Toona.

Once you catch the Toona, return to the Old Wayfarer again and your next target
will be the Loovar. After you catch all three of those fish, visit the Old
Wayfarer once more and he'll give you the Big Catch Lure. Now you can catch the
last three fish, but they're harder to catch and they're much bigger. Once you
catch the Rusty Swordfish, take it to the Old Wayfarer to get a Ship Part.
Catch the Neptoona he mentions to you, then take it to him and he'll give you a
Heart Container. Finally, catch the Stowfish and take it to him to get another
Ship Part. Here's a list of the six types of fish in the game.

so get an heart continer!