Fish farming is an easy method of catching large quantities of fish and other items by fishing. Most AFK designs involve right-clicking an iron door with tripwire above it, causing fish caught to flow into a hopper and then into a collection chest.
AFK Automatic Fish Farming
Automatic fish farming involves right-clicking a door with tripwire above it and hoppers for collection. Its a good idea to use an automatic fishing rod dropper to supply new fishing rods when the previous one breaks. The ideal rod is Lure 3, Luck of the Sea 3, Unbreaking 3 and Mending. Apply a rubber band over the right mouse button to make the experience truly afk.
Note: None of these work in Bedrock Edition, due to how tripwires and doors work.
It also helps explain the mechanics of AFK fish farms.
Docm77's Realistic Fish Farm
This farm is very useful, because it does not use fishing and can generate for you without having to hold down a button. It works using Soul Sand making Bubble Columns heading up to a cobweb layer, which looks like a fishing net and catches the fish, then they slowly suffocate and drop down onto the water conveyor to some hoppers.
This farm doesn't work anymore as of version 1.13.1 because the fish don't die in the cobwebs anymore.
Alternative Fish Farm
A modification to the farm that does work in 1.13.2 can be made by outlining a 2x2 or 3x3 bubble column of water with hoppers and cobwebs on top. The fish will land on the outside wall and suffocate in the cobwebs.
This design uses an Iron Trapdoor instead of a door.
To use an AFK fish farm, the player will have to hold down the right-click button. When the rod is cast, it will trigger the tripwire, opening an iron door. When a fish is caught, the bobber will suddenly drop down, thus closing the door and breaking the line. This forced the rod to reel in whatever was caught. Beyond the iron door, there is a note block to absorb all the other right-clicks that don’t result in anything.
Semi-automatic fish farming (not designed for AFK)
Semi-automatic fish farms involve detecting the fishing rod using tripwire to close a door while fishing, improving fishing rates. The next video is an example:
This is a basic design using hoppers to automatically collect fish while being very quick and simple to build.