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, with an automatic fishing rod dropper to supply new fishing rods when the previous one breaks.
Automatic fish farm:
More compact variant:
This next one uses an iron trapdoor instead of an iron door.
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.
This is a basic design using hoppers to automatically collect fish while being very quick and simple to build.