Minecraft wiki entry for Mob_farm, associated text: For a few cases (notably Overworld gold farms), a 27×27 water tray (counting the walls!), with two levels, may be needed. Start with a classic 9x9 pan surrounding the drop chute. Move up a level to surround it with the rest of the large pan — slabs or blocks suffice for most of it (put real blocks at the boundary, so you can place water on them), but the outer ring should be a 25×25 square of fences on the same level (that is, you'd have a half-step up from the outer pan to the fences). Surround this with the 27×27 outer wall. The inner pan gets a water source at each corner, which should send the flow in to surround the hole. For the outer pan, water sources go every other block on top of the fences (place them against the outer wall), but not in a complete ring. Place the sources every other space along two opposite walls; the water should flow exactly to the edge of the inner pan. Now, if you fill in the entirety of the other walls, you get extra water sources forming from the corners, and your current becomes a sheet of mostly-still water. For the other two walls, put only five sources each covering the middle — that is, one at the midpoint, and two on each side (still every other space). This sends the water flow to the other edges of the inner pan.