I took the highest dose recommended on a high quality brand of fish oil (a higher dose version) while taking bipolar meds. I took it more to lower my triglycerides than as a moodstabilizer medication. It neither lowered my triglycerides nor seemed to make a huge difference with my moods. It was more just a pain in the butt.

I think one would usually need prescription fish oil to possibly see a difference. If someone took more than the recommended highest dose of OTC stuff, I'd talk to a pharmacist or doctor about that. I, personally, think that if fish oil worked sufficiently for people with bipolar disorder that it would be frequently prescribed for it, but it isn't.