Just a quick reply, we can farm species of fish that like promiscuity (and there are quite a number of those, contrarily to the land animals we eat, that may like to hang out in groups but not to the point of stepping on each other's hooves/claws). We can also focus more on vegetarian fish species, like carps, instead of focusing on fish that need to be fed with wild fish (which is currently a common scenario in aquaculture, to the rate of >1 kg of wild fish per 1 kg of farm fish produced).

The main argument in favour of pisciculture is that the current rate of trawler fishing is expansionist while edible fish stocks in the oceans, are, well, diminishing.

Pisciculture may not have to be a permanent feature, but could be construed as a temporary measure, alongside a moratorium on massive fishing that could be implemented to give the ocean some time to replenish itself, after which better fishing regulations could be implemented.

This is just off the top of my head, I will have to write a diary on this after studying this issue some more. Maybe I'll end up with the conclusion that eating fish is evil too, though for now the issue about whether fish actually feel pain or not has not yet been unambiguously resolved.