Minecraft wiki entry for Axolotl_farming, associated text: Water source blocks occupy the bottom four layers of the room. The fifth layer has a row of water source blocks on one side, and one more opposite the hopper, to create a water flow that pushes items toward the hopper. It can be tedious to get this top layer of water flowing properly, because water on top of water does not spread like water on a solid surface. Place easily-breakable blocks like slime blocks or dirt under the end of the flow to force it to spread, then break the block and repeat until the water is fully spread out. It helps to have a Respiration effect active while doing this.