Eating a "ton of fish" does not necessarily mean you're ingesting an optimal amount of EPA/DHA.
It could, if you're choosing wild salmon, sardines, and cold water species. But not tuna.
Also, that's a lot of tuna to be eating. Not trying to turn this into a nutritional voodoo thread, but the mercury levels in tuna are well-documented. Have you ever had your mercury levels checked?
Flax's conversion rate to EPA-DHA is limited in many individuals. Ground flax seed is still nutritious, just not a great source of EPA/DHA.
Taking fish oil to me is an absolute no-brainer.