Tilapia may well be fine as they are largely fed a plant based diet.
Salmon are fed a fish based diet. The fishmeal is obtained from less desirable fish but they're still taken from the oceans. Their stocks won't be as dangerously low but eventually they will be consumed.
I was looking into it a while ago - remember reading that fish farms can be quite terrible for spreading disease and polluting the surrounding area. Not to mention farmed salmon being fed dye etc.
Sustainable just means they can keep on doing it. Not that it's environmentally positive.
Actually went to Hawaii for the first time last week caught and caught few Yellowtail and skipjack tuna of the Kona coast. They fight like hell. Ate it on the boat.