Minecraft wiki entry for Guardian_farming, associated text: If the design is to kill guardians with fall damage or to have the player AFK near the bottom of the farm, all undersea caves (and any nearby islands) within a 128-block radius will need to be lit, slabbed, or flooded to prevent other hostile mobs from filling the mob cap. Alternatively, an AFK spot high above the monument (Y=160 to Y=170) may be used so only the monument is within the 128-block-radius sphere.