Yeah. But a lot of thier nipping is just to see if something is food or not. I've seen one looking at a stationary fish and jockeying a round as if to look at it from all angles ,then heasitate... moves forward looks at it....backs off... looks conflicted as if fighting temptation; then "oh what the he**" Chomp! at which point the heretofore stationary fish takes off like and the puffer looks startled and all innocent "...What? What happened?"" I didn't do nothin!"
Anyway, two problems I've seen are excessive tooth growth and a blackening of the belly and lateral area where it is normally white. I've tried to feed them snails and other hard shelled food to wear them down. But MTS snails are not tough enough. I've been told some people anesthetize them with a couple of drops of clove oil in a gallon container and clip them with toenail clippers. I don't know but if the teeth grow to big thier jaws don't work right they cannot eat and thee is danger of infection.
The blackened belly thing is another Thing I am trying to get more info on. It can dissappear in minutes but seems to be abnormal. And none of the remedies I have tried are effective. antibiotics, tea oil, aquarium salt, methyl blue malachite green and so forth.