The 'Boyfriend' hitmaker - who is dating singer-and-actress Selena Gomez - admits snogging the deceased aquatic animal for a bet was the "worst dare" he's ever done in his life.
He said: "The worst dare I ever did was kissing a fish. It was dead. It was pretty gross."
When he is not busy kissing deceased animals, the 18-year-old singer has found time to tone his muscles in an attempt to improve his physical and mental strength.
He explained: "I've started working out more and getting a bit heavier. I'm just staying fit and active, and want to be strong physically and mentally. It's good for touring. You've got to have agility."
While he has been doing pull-ups and chin-ups, Justin admits he is not yet in peak physical condition and gets "tired easily" when he is working out, but he hopes to rectify that by going out for runs "a lot more" often.
He added to We Love Pop magazine: "I also do pull-ups and chin-ups. I can do about 25 in a row. I do need to run more, I get tired easily.
"My cardio isn't what it should be."