Aquaponics combines hydroponic plant production with aquaculture (fish farming). Fish (usually tilapia, trout, or catfish) produce waste that nitrifying bacteria convert into nitrates, which plants absorb — cleaning the water in the process. It's a closed-loop system using a fraction of the water of traditional farming.
Aquaponics is capital-intensive and operationally complex (two interdependent biological systems) but yields both fish and produce from a single footprint. Small-scale commercial aquaponics is often used for restaurant-direct leafy greens paired with specialty fish.