Buy sustainable or organic
If you choose to eat fish you need to consider few factors. Wild caught fish had a natural life but usually suffer throughout the capture process and are nearly always killed inhumanely.
Farmed fish are often kept in unnatural conditions with little respect for their natural behaviour. They live in high stocking densities which can cause bad water quality and parasitic infestations. Some of farmed species are killed using humane methods (all Soil Association and RSPCA labelled fish) but many are killed inhumanely (eg seabass and seabream within the EU).
Carnivorous species like Salmon, Trout or Seabass are usually fed with feed containing a big proportion of fish oil and fishmeal. To obtain those products large quantities of small wild caught fish are needed. Again, these are not caught humanely.
Look for the Marine Stewardship Council logo to ensure wild-caught fish is sustainable. MSC unfortunately do not have welfare criteria listed in their certification.
If you do buy farmed fish, buy Soil Association organic for higher welfare standards. Otherwise fish may have suffered in over-crowded tanks, experienced unacceptable periods of starvation and been slaughtered inhumanely.