Fish oil contains high concentrations of EPA & DHA (the best fatty acids one can consume). Your body can convert the omega 3's in flax seed oil into them, but not very efficiently. If you have no other abundant source of omega 3's in your diet, you need 3 tbs of flax or 6 grams of fish oil for an optimal intake of omega 3 fatty acids. Taking 6 grams of fish oil with a little extra flax will not hurt of course. Just bear in mind that when carbs get extremely low, one should generally take in a considerable percentage of their calories in the form of fat, so if you need to ad fat to your diet, flaxseed oil is an excellent choice.