Fish Oil and Omegas

There are two primary types of omega fats: omega-6s and omega-3s. To complicate things further, there are three types of omega-3s: alpha-linolenic acid (ALA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA).
Omega-3s are considered essential because they cannot be manufactured by the body and must be obtained through food. I’m going to spare you having to read thousands of studies on omega fats (there are currently 11,084 listed on PubMed), and give you some benefit highlights:
Anti-clotting properties
Lower the risk of heart disease, including coronary heart disease and atherosclerosis
Lower blood triglycerides
Lower blood pressure
Help alleviate mood disorders
Help people with ADHD and dyslexia
Improve memory and learning skills, and reduce the risk of Alzheimer’s disease
Help reduce the affects of inflammatory skin disorders such as eczema and psoriasis
Help alleviate rheumatoid arthritis
So the question that remains is, how many omega-3s do we really need? Fortunately, there are some studies for us to reference.

Knapp HR, Fitzgerald GA. The antihypertensive effects of fish oil: a controlled study of polyunsaturated fatty acid supplements in essential hypertension. N Engl J Med. 1989;320:1037-1043. JANA 28.

The study reported that blood pressure dropped significantly in fifteen hypertensive subjects given 15 grams of fish oil per day.

Here's another:

Morris M, Sacks F, Rosner B. Does fish oil lower blood pressure? A meta-analysis of controlled trials. Circulation. 1993;88:523-533.

This study look at the combined results of thirty-one different fish oil studies and concluded that fish oil does reduce blood pressure, and the larger the dose, the better the results:

Less than 4 grams of fish oil daily = no change

Between 4 and 7 grams daily = a drop of 1.6 to 2.9 mm Hg

More than 15 grms per day = a drop of 5.8 to 8.1 mm Hg

What to Look For In A Fish Oil Supplement
The major concern with fish oil is its tendency for rancidity.

The fish oil manufacturer should add vitamin E to prevent oxidative damage to the oil so that it remains stable during transportation and while on the shelf.
Purity should be independently certified
For optimum absorption and bioavailability, omega-3s should be a in a triglyceride form