Aquaponics is the way to grow the most food in a confined setting. So if there is demand for 'locally grown' produce in cities then entrepreneurs will fill that niche - more expensive but local produce. It is an interesting example, and I have read about the people that have used their backyard pool with a chicken coop over it so that the chicken waste fell into the water and the tilapia ate it.

I see it all as just a niche type system for areas that have no arable land. If arable land is available, it is much more cost effective. Most people buy the cheaper version of an item.