This New York Times article
from 2011 lists several reasons why farmed tilapia may not be the best thing in the world. While farmed fish reduce the need to overfish wild fisheries, fish waste from the farms has a big impact on the ecosystems around them. Additionally, while tilapia provides the lean protein doctors want you to eat instead of red meats, the beneficial fatty acids are almost completely missing. In fact, tilapia contain higher amounts of a different fatty acid which may actually be harmful. In any case, it's an interesting article and look at this quote:
The growth has been abetted by the creation and marketing of new products like fleshy “tilapia loins” — even though fish do not possess that anatomical feature.