Krich was right about piranhas being the clear answer. Tom's idea about a shoal of rhoms gives me anxiety...I think that might be as close to fearing your own aquarium as one can get...DO IT, lol.
However, this lfs in LA had a nearly 2 foot porcupine puffer (this thing was huge) that bit off some guys entire finger after he stuck it in the tank to touch the puffer. From the outside it looked cute and friendly, following your finger along the glass...he didn't realize they have some serious chompers and obviously didn't read the huge signs "FISH BITES - DO NOT PLACE HANDS IN TANK". Had to be taken away in an ambulance and tried to sue the store for losing his finger but the store had video of the incident with the huge warning signs visible. Needless to say, he lost the case and his finger.
Mine crunched crab legs at 3 Inches. I can't imagine the power of a two footer.
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