The reason that farmed salmon is considered bad is because the farmed salmon could contain contaminants such as carcinogenic dioxins and PCBs.
Farmed salmon are fed unnatural foods as well that can contain special hormones that make the farmed salmon grow faster.
When the farmed salmon don't eat naturally and grow to fast they can be more unhealthy for people to eat.
However farmed salmon is not all that bad and you're not likely to get any health issues or cancer from farmed salmon.
But if you want the healthiest salmon it's best to eat wild caught salmon but wild caught salmon costs more than farmed salmon because it takes more work to catch wild salmon than it does to raise and catch the farmed salmon.
Fresh wild caught salmon contains a much healthier ratio of anti-inflammatory omega-3 fats to inflammatory omega-6 fats, as well as an overall better nutritional profile.