I really have not heard of this laxative action, esp with only one fish oil...

I had a patient taking 13 of those a day..and he had no problem.
(his doctor put him on that for an autoimmune kidney inflammation..based on
a study from China).

I have however, had looseness from strong Evening primrose supplements.

There is only one gram in that fish oil... not much.
I have had people (myself included) get loose from olive oil, esp Extra Virgin, if it is too liberal on a salad. In Italy they use a tablespoonful of Extra Virgin for
a laxative sometimes. I had a kid with a kidney transplant once whose mom wanted a gentle laxative (I was leaning toward sorbitol sweetener in dietetic candies---but Grandma from Sicily won out with her olive oil! ).
Sugar free PEZ landed my son in the can for almost 1/2 a day once! Beware sorbitol in sugar free candies!

Yes, Mel, I remember the Metformin effects too... not pleasant!