Found this and thought I'd share seeing as how this subject comes up a lot.   I just want to add that the dosages listed are a guideline and each product is different, so if your dog has a compromised immune system or other skin issues your dosage may be higher.

Omega 3 fatty acid supplements for dogs: There is a huge difference in the omega 3 - omega 6 balance in  commercial dog foods and the omega 3 - omega 6 balance in the diet of wild carnivores.  The wild animal’s diet is composed mostly of small animals and birds eaten whole.  This natural diet is high in omega 3 fatty acids and low in omega 6s.  Commercial dog food is exactly the opposite.  Dog food contains a lot of cereal grain which is high in omega 6 fatty acids and low in omega 3s.  This lack of omega 3s affects the health of our pets in ways that nutritionists and veterinarians are only beginning to understand.     Omega 3 fatty acids are vital for normal function of every organ system in the body, particularly the immune system.  A lack of omega 3s (or relative excess of omega 6s) contributes to allergic and degenerative disorders, causing: Joint inflammation and arthritis   Allergy - dry itchy skin, recurring skin infection, recurring ear infections   Autoimmune disorders such as inflammatory bowel disease   Loss of mental alertness in older dogs Flax and flax oil are good sources of omega 3s but unstable. Heat processing destroys most of flaxseed’s value. Cold-pressed flax oil is an excellent choice, but has a short shelf life and must be kept continuously under refrigeration.  Flax is an inactive source of Omega's and unless your dog is able to convert it it is of no real value and you should stick with fish oil. Oils from cold water fish  are an excellent source of omega 3s and a more practical  way of supplementing our pets’ diets: Deep water fish concentrate toxins like mercury and cancer-causing PCBs. If we are going to use fish oil for ourselves or our pets, purity is an important concern.    Fish oils oxidize easily. Old or improperly stored fish oil oxidizes, creating free radicals which are toxic. Pure, fresh fish oil is light in color. It has very little fishy flavor and almost no odor. The pump dispensers used for some veterinary omega oil supplements are easy to use, but oxidize the product. This isn’t what you want. Once opened, fish oil should be refrigerated and used within three months. Wipe away excess oil from the lip of the bottle after each use so you don’t accumulate oxidized oil and contaminate the next dose.   The lack of Omega 3 and the excessive amount of Omega 6 (up to 20 times the amount of Omega 3) in the modern diet ultimately create degenerative conditions. The store-bought processed foods still do not supply an adequate amount of Omega 3 to properly balance the ratio. (The ratio should be only 2 times the amount of Omega 6 to Omega 3.) And many nutritional supplements are still including the Omega 6 (LA) fatty acids -exacerbating an already severely imbalanced and unhealthy situation!! The best solution to insure our pet's health is to provide a high quality Omega 3 Fish oil supplement along with a premium food. Also, Cod liver oil, while it does supply some Omega's to the diet, is also high in Vit A and D and in reaching the right levels of Omega's from Cod Liver oil you are likely to be overdosing in A and D.  You are much better to stick with actuall fish oils from cold water fish, which are derived from the fatty flesh of the fish.

Hmmm....I wonder if this is the same for humans. I am a carboholic, so maybe I'm getting too many Omega 6's. I have high triglycerides and I'm supposed to start taking an Omega 3 supplement, but I've heard they cause fishy tasting burps.