Mayo recommends a dose of no more than 3 grams per day. Side effects can occur above 3 grams or less depending on the patients condition.
Richard has good advise when saying that the oils need to have the metals removed as fish generally has a high level of metylmercury and the organic compound of mercury, that remain in the oils, in lesser quality brands.
A big problem being found now is that Harvested fish tend to have high amounts of "other pollutants" that are being found upon testing.
As a supplement the QA is not well regulated, and although "distilled" (a general term) many pollutants will remain in the oils: Some even potentiated due to the heating processes.
Many physicians are backing off on recommendations for OTC fish oil supplements, changing to the more regulated medicinal form. There is a big push back from the insurance companies about this as it increases cost.
Nutritional groups advise that patients should refrain from OTC supplements and change to eating fish products 2 times a week. Dietary intake allows for the ingestion of DHA EPA and also ALA. ALA is also found in fish oils in a limited form and is converted to DHA or EPA in the body during digestion.
Multiple research studies show that dietary recommended intake may be enough to give the protection that supplemental usage does. This provided that dietary intake is of good quality foods.
Rare but recent, you will begin to see Omega Fatty acids in Oils being released that are derived from other food grade items. Specifically Orange Juice Eggs and Algae.
Not all physicians are on board yet, but as more publications are released, I suspect there will be a trend in physicians changing to "algae" caps or "seaweed"
The public just do not like those words.
++++Although you would be surprised how many of the American staple foods have some type of seaweed or algae in them...
Funny to read this as I had been working a bit recently with the safety and efficacy of omega 3 oils in another product...
Not sure if this helps but here is the webs most popular photo about eating fish... Literally.
https://encrypted-tbn0.gstatic.com/image...zCdcV7oAEG