Flarefin Koi

Nothing about this catch is casual. A normal line dropped in lava does nothing at all, so you need gear that survives it: the Lavaproof Fishing Hook, the Hotline Fishing Hook, or a pole baited with a critter caught down in the Underworld. That requirement is the real cost, not the fish.

You do not have to camp hell to do it either. Any lava pool big enough to fish works, including one you dug and filled yourself, so a lot of people set up a small pit near their base and farm it in comfort.

The Inferno Potion is the reason to bother. It burns everything around you while you concentrate on dodging, which is exactly the kind of damage you want during a boss fight where you have no spare attention. The alternative use is a Seafood Dinner if you would rather eat your rare catches than throw them.