Antibiotics of all sorts, runoff, etc... There's no magic panacea. Thai and Vietnamese shrimp farms have been known to kill of swaths of mangroves. This you can find from a Google search.
On a more anecdotal basis, I've also heard that Norwegian salmon is fed quite a cocktail of antibiotics etc and some people for that reason prefer New Zealand or Scottish salmon. Again this part I have no evidence.
So anyway my point is that it's definitely not as easy as it seems unless I misunderstand what "land based" means, I just assume they're fish farms not in the open sea but somewhere in a lake or an artificially made pool.
