Armored Cavefish

The reason to bank these is the Endurance Potion. Flat damage reduction stacks on top of everything else you drink before a boss, and it helps equally against one big hit and against chip damage from adds, which is not true of most defensive buffs. Anyone dying to contact damage should be carrying it.

A Seafood Dinner is the other use, and it is a fine thing to cook if you have fish spare and nothing to fight.

Any pool below the surface works and the biome does not matter as long as it is not a Desert, so the easy routine is a small stocked pool beside whatever underground base you already keep, cast between other jobs. They are not a common pull, so treat it as something you top up over several sessions rather than in one sitting.