Fish farming is an easy method of catching lots 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.
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.