The raising of fish! Half the equation of Aquaponics.
Maintaining a tank is relatively easy. When you get into breeding, and the specifics of many species, their interactions, and how you can play them off each other, this gets fun and complicated. There are, though, some well understood, tried-and-tested systems (ie raising mixed-gender tilapia together with largemouth bass to allow the tilapia to breed prolifically, while the bass to eat all their fry. When you want more tilapia, you simply separate a couple tilapia for a while).