Omega-3 fatty acids in fish oil are available in 3 forms

Non-concentrated Natural Triglycerides (TG):

Triglyceride is the natural state of omega-3 in fish oil. Essentially all fats and oils in animals and plants are in triglyceride form. Natural triglyceride fish oils provide a maximum of 30% of omega-3s (30mg omega-3 per 1,000mg of fish oil), while another 70% is other fats. This is the most common form of fish oils on the market

Concentrated Ethyl Esters (EE):

To concentrate the amount of omega-3, the oil has to be converted from its natural triglyceride into ethyl ester form. The ethyl ester concentrate is commonly marketed as “fish oil concentrate” and typically contains 50 to 70% omega-3s. Nonetheless, the human body has difficulty processing and absorbing the ethyl ester compound, which is not found in nature

Concentrated Re-Esterified Triglyceride (rTG):

The most recently developed rTG breaks down the EE form of omega-3 and reconverts the free fatty acids backs to its natural triglyceride form. Since rTG omega-3 is identical to natural triglyceride form, it is better tolerated with less gastrointestinal side effects. It is also better absorbed and utilised easily by the body as compared to omega-3 in EE form.

A great majority of concentrated omega-3 fish oils in the market are in the EE form, whereas a limited number of omega-3 fish oils are in the super-absorbable and bioavailable rTG form. This is attributed to the extra costs involved in re-esterifying ethyl ester fish oil back to rTG form, and it is not surprising that rTG fish oil is more expensive than EE fish oil.

¥Prostaglandins Leukot Essent Fatty Acids.83(3): 137-141 (2010)

In brief, look for re-esterified triglyceride (TG) rather than ethyl ester form of fish oil for better absorption, utilization, stability and potency.