Minecraft wiki entry for Axolotl_farming, associated text: The diagram below illustrates the concept. The spawning room area is 8×8, and 6 blocks high. The 8×8 dimensions were chosen because it requires only one hopper, but different room sizes and shapes are possible with a carefully planned arrangement of flowing water and hoppers. The hopper is needed only to collect glow ink sacs. If you don't care, then don't bother with the hopper.