The brown pee is supposed to be bromine, or could be due to dehydration
I've never had noticably brown pee myself even on over 100mg doses
I have taken fish oil from what I regarded as a good well tested pure source
for a few years now, I know colitis sufferers have got benefit from fish oil in quite large amounts and I think it did improve my symptoms.
If there was any significant mercury in this I doubt they would have seen the benefit,
I do believe mercury is a significant contributer to colitis if not the cause in many cases so I'm not too concerned about the fish oil I use