The Goldfish Myth mod adds a single new fish to the game; a Goldfish. If you catch one while fishing you do have the choice to cook and eat it:

Or if you throw it back into a large body of water then you will be granted good luck for a limited amount of time. And the luck will stack up to luck III if you throw multiple goldfish back. This means that Looting will increase up to 25% whether you have the Looting enchantment or not! But if you do then the Looting is enhanced even more.

And it will also help in your mining as well. Once you have your luck buffs, all gem mining has a chance to double and even more chances if you have the fortune enchantment!

Happy fishing!