I have used the scroll of terror on this room before and it works quite well as long as you don't accidentally attack the fish. Sometimes they will crowd the door trying to flee, but walking close to the door will frighten them off in a different direction (assuming there is space to flee).
I actually go with a different strategy, depending on class, but I mostly play as Mage And Huntress.
Mage: Throw a Dart into the doorway, as it will open the door. Then, use your wands to damage the pirhannas. If hiding behind a wall, take a little damage and come near door so they get in sight range.
Huntress: Same Idea with the Dart, but use your boomerang instead. Free meat!