Be sure to understand the difference between fatty acid ethyl esters (FAEE) and triacyl-sn-glycerol (TAG) based formulations. The ethyl esters are not "fish oil". Most of the ethyl esters of omega-3 are derived from natural fish oil. The conversion of natural fish oil to ethyl ester derivatives takes place by boiling natural fish oil in the presence of ethanol with sodium ethoxide (a catalyst) that promotes a transeterification reaction. The three ester bonds of TAG bond fatty acids are broken and converted to single mono-esters known as fatty acid ethyl esters (FAEE). So, although the labels of many "fish oil" dietary supplements claim they are "fish oil" in reality, the liquid is not the oil in fish.