Golden Carp

Most early money plans come down to selling something you cannot use, and this is the single best thing fishing will hand you in that category. One catch beats a full chest of ordinary fish. It is also rare enough that you cannot build a plan around it, so treat it as a windfall rather than income.

Any underground or cavern water outside the desert can produce one, which means the pools you are already fishing for crates count. There is no special setup and no bait tier that improves your odds beyond what you would already be using.

Later versions made it an ingredient in Golden Delight, and that is the only reason to hold one instead of selling. Everything else in the dish is a critter you catch with a net, so the carp is the part that gates the recipe.