This is a one-day mod i did to take a break from a larger mod... It only adds one new type of fish. Catch it with the normal fishing rod. It will give you minimal saturation even when cooked. Instead you may choose to release it back into water. Maybe those stories about releasing the goldfish turn out true after all.
Details are in the spoiler block below. I do suggest you figure the effects out on your own...
Releasing the goldfish will bestow a long-lasting (configurable duration) luck effect upon you. Luck effect is stackable - throwing more goldfish into water will give you higher levels of luck effect, up to a maximum level of 3 (configurable).
Get you sword... because each level of luck (up to L4) gives you 25% chance to increase the looting level by 1 (works even if you don't have looting). Each level from 5 to 8 gives 25% chance to get +2 to looting (or a +1 if you fail a roll). This feature can be turned off completely.
Or... get your pickaxe.... because each level of luck adds a little to a chance to double any gem that drops from a block. Works for gems only (modded or vanilla). Synergizes strongly with fortune enchantment (as there will be more stuff to potentially double). This feature can be turned off completely.
Note: throwing a goldfish into the puddle won't work; it will know whether you released it into a decent body or water or not.
I'll see about compatibility with mods that add custom fishing rods (mods which use vanilla rods are surely compatible).
Other than that, nothing. This is feature complete since first release.