Use a Mariculture rod (reed rod works) to fish up Cod, smelt them in the Mariculture Crucible for an 11% chance at leather. If you want to you can breed them, or attempt to breed up to Perch (Minnow + Cod = Goldfish, Goldfish + Cod = Perch, Perch + (Goldfish I think...) = (something with a...