The first fish I ever owned was a Brazilian puffer. Even though I fed him his proper diet, his teeth still grow too fast. Every three to four months I had to clip his teeth so he could eat properly.
Heres what I used;
-One bucket of tank water
-one bucket of tank water mixed with clove oil. (one drop per 3 cups of tank water)
-one set of nail clippers
Clove oil can kill a fish. So it's a good idea not to mess up the measurements. Better yet research it. You dont want to risk your fish's life on one persons suggestion on the net.
I simply pulled him out of the bucket with regular water and put him in the clove water.
Then after he turned belly up ( takes about 30 seconds) I pulled him out and clipped his teeth.
After I was done clipping, I would put him in the bucket with just tank water.
After a good half hour of recuperating I would drip him back into his tank.