Almost any fish that doesn't fit into there mouth, and is not big enough to eat them.
Not always true I had a wild congo bichir I dropped in a 100 gal with veil tailed albino oscar and a green phantom pleco and the little bastard took em both out first the phantom went out with two bite marks on both sides of its head then three days later the oscar died from a bacterial infection from being biten for the same ammount of time
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