OTOH farm raised fish are often quite detrimental to the environment in other ways - unless it's a sealed system (which is only practical for smaller fish), they add massive amounts of nutrients and pollution to the local environment. Also, many farmed fish are fed smaller fish, which are caught wild, and require some 3kg+ of feed fish to produce 1kg of farmed fish.

This is a problem with pretty much every type of factory farming. Food can be cheaply imported from any country around the world but the animals still produce feces locally. The factory farm owners do not own any land to use this natural fertilizer on. Therefore it is either stored in lagoons or in case of aquafarms just sinks to the bottom of the water until years later it becomes impossible to farm fish anymore because of excessive algae blooms.

Fish farms are future superfund sites.

Farm-raised fish are often treated with antibiotics, which means you have less plastic pollution but more antibiotics-overuse problems.

farm-raised fish cause plenty of havoc in the ocean.

They could be on land: http://www.uarahfisheries.com.au/