This can be done once you have an aquarium and any desired fish to clone. Once you have the fish to clone, assure it is in the first slot of your inventory outside of the aquarium. Pick it up with Square, then repeatedly use Select to sort your items menu until the aquarium is in that first slot. Then press Circle and Square in that order - you'll still be holding the fish, but a clone of it will also be in the aquarium! This can be used to clone fish to cook for healing, to sell for money (though the Lure glitch is faster), and to just making breeding a bit easier.You can also more easily win Fishing Contests as you only need to catch a big fish and clone it twice as opposed to catching three separate big fish.