Ultra-purified, pharmaceutical grade, molecularly distilled. Over-the-counter vs. prescription. Gelcap, liquid, emulsion.
There's a mind-boggling variety of choices in fish oil today. A visit to any health food store, or any "big box" store for that matter, will yield at least several, if not dozens, of choices, all with varying and often extravagant claims of purity and potency.
So what's the real story?
Given the analyses conducted over the years, along with my experience with dozens of different preparations, I believe that several conclusions can be reached about fish oil:
Fish oil is free of contamination with mercury, dioxin, PCBs, or furans. To my knowledge, only one fish oil preparation has been found to have a slight excess of PCBs. (This is different from cod liver oil that has been found by one source to have a slight excess of PCBs.)
Oxidative breakdown products differ among the various brands. Consumer Lab (http://www.consumerlab.org/), for instance, has found that several widely available brands of fish oil contained excessive oxidative breakdown products (TOTOX). You can perform you own simple test of oxidative breakdown products: Sniff it. Your fish oil should pass the "sniff test." High quality fish oil should smell non-fishy to lightly fishy. Rancid fish oil with excessive quantities of oxidative breakdown products will smell nasty fishy.
FDA approval does not necessarily mean greater potency, purity, or effectiveness. It just means that somebody assembled the hundreds of millions of dollars to obtain FDA approval, followed by lots of marketing savvy to squash the competition.
This means that there are a number of excellent fish oil products available. My favorites are the liquid fish oils from Pharmax, Nordic Naturals, and Barleans. Capsules from Carlson, PharmaNutrients, and Fisol have also performed consistently. The "big box" capsules from Sam's Club and Costco have also performed well and are wonderfully affordable.