Water flush mob farm

KelpMC's ocean mob farm, 22 stacked spawning platforms of slabs with dispensed water flushing each one into a central drop tube and a 5x5 hopper floor feeding thirty chests. Everything runs off one lever. It is the largest thing in the library by block count, nearly 190 stacks of slabs on its own, and unusually for a farm this size the stated rate holds up. The one thing the video gets wrong is the layer count, which several builders found is better lower.

• Put campfires on top of the hoppers. This is the most repeated tip on the video and it fixes chicken jockeys, witches, armoured mobs, baby zombies and spiders surviving the fall. Perfectmanxxxxx adds that from 26.1 the zombie leader fix gives some zombies more health so the drop alone no longer kills them, and CurtisD1986 found the hoppers full of live armoured mobs after six hours without it.
• 22 layers is too many. firuletecosmico962 runs 10 and says the bottom to top spawning order wastes the upper platforms, luka0090 found not all of them stay loaded, and HedgeHogDino13 got 13,000 an hour from 10 layers alone.
• wombat6 builds the whole thing two blocks higher so an item sorter fits under the hopper floor while staying above water level.
• VibhorKabra and farmcrafter1og put nether portals in the drop tube so the mobs die in the nether, which clears the mob cap faster and raises spawns.
• ponder9011 uses moss as the temporary block, which is the cheapest thing to remove afterwards.
• For Bedrock, justinpatrick4569 rewires it with a redstone clock driving a sticky piston and redstone block into the dispenser chain, and patraomathheww4886 puts mangrove roots between the observer and the dispenser.