The reason that farmed fish are considered bad for your health is because they contain less Omega 3 and since they contain lower levels of the Omega 3 that means that the farmed fish are less healthy and less able to help prevent heart disease, improve brain health and other diseases.
Farmed fish are also usually treated with antibiotics which can make the fish less healthy when they are farmed fish.
But still farmed fish are not all that bad for your health and the farmed fish are still very safe for you to eat.
Fresh fish that come out of a lake, river etc are much healthier for you because they are not fed artificial foods and other vitamins.
But farmed fish are not really all that bad.