This is a secret extra for the Crypt Of Decay (Episode
2 Map 3). After the spiked hall, go forward, turn backwards, then
look to your right. There is a room there where you must take
a leap of faith to cross. Get the quad and keep going towards
the wall until you reach a hall. Enter it and go down and swim.
The Dope Fish is at this location. Go to the left of the fish
and shoot to find the exit to the secret level.
Once you find the Well Of Wishes, go down into the well and look
on the wall. The big green stupid looking fish with uneven buck
teeth is the Dope Fish from the Commander Keen series of
games.