Mahi Mahi is a good choice - it's not a carnivore, therefore it's less likely to be affected by the biomagnification of the food pellets they feed to salmon and other carnivorous fish.

Biomagnification occurs when you feed meat to a carnivore high up on the food chain. All that meat comes from other animals that have already ingested pollutants such as mercury. When your farmed salmon/carnivore eats the pellets of other fish, its eating all the accumulated toxins from the flesh of the reconstituted flesh bits, so a "pure" farmed fish is just as hazardous as a wild fish.

The other reason to avoid farmed salmon is that its food is gotten using huge factory ships that scoops all the oceanic life off the seabeds, destroying habitat and consuming all the fish that were once the livelihood of local fishermen.