I'm playing the FISTS path.

Hint 1 of 4:

Have you gotten through the doorway with the stone slab?

Hint 2 of 4:

Keep exploring this area until you've taken care of Arnold.

Hint 3 of 4:

Then pick up the amber fish by LOOKing AT ARNOLD.

Hint 4 of 4:

Use the fish in the same room.