In my opinion, the source of the fish oil is very important. Some companies purchase oils from another source. This source may be purchasing fish that is caught in a unsafe area where pollution is high. Some essential oil companies such as Young Living know where their supply is caught and carefully checked.
Also, the delivery pill can make a difference. Most of these oils are broken in the stomach. Young Living Omega-3 is release in the intestines. This might be a consideration for discomfort experienced by some fish oils.