Yeah there are ways to keep the ecological impact low but I doubt we will see them for a while.
Having all land based fish farms would be great as opposed to ocean rearing pens. This way the escapement problem is taking care of and you can disenfect the water being used before it gets into rivers, oceans, etc.
However this is much more expensive to set up that just throwing a bunch of smolts in an ocean pen and throwing in food pellets every once in a while.
Unless there are worlwide regs mandating land based fish farms the worl fisheries could be in trouble down the road.