It also depends on what brand you buy, what types of fish the oils are from, are they freshwater/saltwater....that affects the quality of the oil.
Alot of fish oils are-in my opinion-bad. Not produced well enough. Poor quality. They have high levels of metals in them. When it comes to fish oils I stick to a high quality brand-I use Barleans, and Health from the Sea. Its from fish that are only harvested for oil purposes, farm raised organic fish-not the left over oils from tuna for canning, for example.
There are good brands, you have to look into what the company says about where they get their fish from. Again, just my thoughts.