Build a water bubble column elevator with soul sand, which passes through a lush cave biome.  The lush cave biome must be within the passive mob spawning radius of 128 blocks.

Tropical fish will spawn in the bubble column, being carried to the water surface to jump out of the water column and end up dying on solid blocks.

This creates an unintentional fish farm, creating a large number of entities.  After 5 minutes you'd see between 40-45 fish.