I have read that fish oil can tend to have an affect on your blood, as far as being a little blood thinning or something. However, I don't think a normal dose of fish oil will have much of an affect on you. I think this would only apply if you are taking massive doses, or are taking it in conjunction with something else that also has a similar effect. I take ProOmega
by Nordic Naturals and really love it. It's great for those who keep very active as well. I don't know that it increases endurance, but it is definitely great for you.