Minecraft wiki entry for Axolotl_farming, associated text: The hopper should be two blocks below the flowing water (one block below the top-most water source block). Placing the hopper just below the flowing water layer (which would be normal for water flowing on top of a solid surface instead of on top of more water) risks items getting trapped under the hopper. Place a button or sign on one wall above the hopper to prevent water from flowing in. Then, when an item is pushed into the corner by the flowing water, it falls one block down into the hopper.