From Quake Wiki
If Rotfishes are even a concern, let them get moderately close and kill them with a shot from the Double-Barrelled Shotgun, that will take care of them most likely.
monster_fish is the entity for placing Fish in a level.
|targetname||The targetname of the Fish. When triggered, the Fish will wake up if inactive.|
|target||The targetname of the entity to be triggered when the Fish dies.|
|killtarget||The targetname of the entity to be removed when the Fish dies.|
The monster_fish entity supports the following spawnflags:
|1||Ambush||The Fish will not wake up from seeing other monsters wake up or hearing sounds.|
It also supports the standard spawnflags for difficulty and deathmatch presence.
|256||Not on Easy||The Fish will not spawn on Easy difficulty.|
|512||Not on Normal||The Fish will not spawn on Normal difficulty.|
|1024||Not on Hard||The Fish will not spawn on Hard difficulty.|
|2048||Not in Deathmatch||The Fish will not spawn in Deathmatch mode.|
This is the format used by TrenchBroom and Worldcraft. Note that the model(...) parameter is not supported by Worldcraft. This does not include the Monster base class definition, which is also required.
@PointClass base(Monster) size(-16 -16 -24, 16 16 24) model(":progs/enforcer.mdl") = monster_fish : "Fish" 
This is the definition format used for most old Quake editors, including the original QuakeEd.
/*QUAKED monster_fish (1 0 0) (-16 -16 -24) (16 16 24) Ambush */