Remember to
eat your veggies...
The Dopefish itself, in-game, simply swam around in an attempt to get to Commander Keen in the Well of Wishes. Getting near him is downright stupid, as the Dopefish is more than willing to eat Keen. The trick is to get some schoolfish to swim around you and distract the Dopefish by making him eat one of them instead of you, giving you enough time to swim by.