Tilapia will survive in water that's nearly mud, at high population densities. Their natural habitat is lakes that recede dramatically, concentrating them and making the water quality poor. That makes them very easy to farm. If you manage the pH and nitrate/nitrite concentrations to ensure the water isn't toxic, they'll put up with very high densities and abysmal water quality (read: recirculating water that a thousand fish are pooping in, with an aerobic digester and perhaps brine shrimp to break it down but not a filter that needs replacing). They also grow very quickly.
Tl;Dr they're cheap and can be flavored any way you please.
(To be clear, I've looked into that chocolate milk water, and I ate a fillet of tilapia earlier today; personally I'm comfortable with it, if we're going to farm animals, we should probably do so efficiently. My feelings on whether that's humane are much more mixed. I'm 99% a vegetarian, this was a bit of a cheat day.)