I take several pills each morning, fish oil being one of them. I'm not certain that each pill I take does anything for me and if I would notice had I stopped taking them. So periodically, I will cease taking something to see if it has any effect. Recently, I omitted fish oil from my morning pills.
I did notice an effect after I stopped using fish oil for awhile, and that was that I had difficulty focusing my attention on complex things and my memory wasn't quite as sharp.