Minecraft wiki entry for Traps, associated text: Dig a 1 block wide, 1 block long, 2 block deep pit with a magma block floor. Replace the 4 adjacent blocks to the air block at the bottom with trapdoors (make sure they open towards the magma so the pufferfish don't escape and die) and open them, then put a pufferfish in each one. Fill the pot with water, and (optional) add an open trapdoor in the hole, which mob AI sees as a solid block. The magma pulls it down and causes fire damage, while the pufferfish poison and damage the mob/player who fell in or was knocked in.