If you kill them as soon as they arrive, they aren't too much trouble, but they swim constantly, and leveling up your swim skill raises your strength and agility. That means that if you leave the little guys alone and give them time to train, they become night invincible.
--and my complements on knowing how to treat a war hero. I wonder if one of the other dwarves managed to gouge it's eyes out, beforehand, or something. Normally, you need a ranged weapon to survive a goldfish encounter, as they drag you into the water and drown you.