Prismite

Keep every one for Lifeforce Potions. That is a top-tier boss buff and the fish is the bottleneck ingredient. Do not burn them on Seafood Dinner: the Princess Fish come out of the same hallowed water, appear far more often, and cover that recipe just as well.

Nothing about the catch requires a broken altar, only hallowed water to fish in. Carry Hallowed Seeds out of an existing hardmode world, grow a patch beside a pool, and you have a pre-Hardmode source. Some special world seeds generate the biome from the start and give you the same thing for free. The potion itself needs other hardmode ingredients, so this mostly matters as stockpiling ahead of time.

They are a rare pull even in the right biome, so go in with strong bait, a decent pole and a Fishing Potion running rather than casting a few times and giving up. Building the pool yourself in a hallowed area is worth it, since a hand-dug hole of the right size beats hunting for natural water.