It does help. In terms of output farmed fish are far and away the most efficient of livestock. It's been a few years, but if I recall correctly farmed tilapia produce 1kg per 1.1kg of feed. That's incredible compared to beef or even chicken.
Also, encouraging well run fish farming operations takes pressure off of the wild stocks. In terms of eating meat, you're doing it right.
A few years ago, I was at the local natural food market and I overheard the manager explaining to a customer that the store did not and would not ever carry farm raised fish. He explained how farmed fish lived in horrible conditions and that the only moral and sustainable option is to only buy free range fish which were caught from the ocean, where fish were part of their proper ecosystem.
From an intellectual, sustainability standpoint, I suspect that you are right and that farmed fish is the way to go. However, this is why I've completely factored guilt and morality out of my environmental decisions - No matter what I do, one of the employees at the natural food store will tell me that I'm killing the planet.
Also, encouraging well run fish farming operations takes pressure off of the wild stocks. In terms of eating meat, you're doing it right.