dogsbestfriend, it's a very beneficial thing to add to your dog's diet, even if you feed a good quality commercial food - they don't contain all that much, and quite a bit of it oxidizes once the bag is opened.
At 12 pounds, one 1000mg capsuleof fish oil per day is fine. Give about 50 IU of vitamin E with that daily, or 100 IU every other day.
gamecockgirl, I wouldn't give that supplement. There is no need to give your dog soy oil, and fish oil is more available to the dog's body than flax oil. The product also makes no statement at all about what kind of fish is used. Stick with a good brand like for example Grizzly Salmon Oil, or any of the products from seapet.com. If you feed home prepared meals exclusively, I'd recommend their "Omega 3 "A, D & E" Oil",it's a great way to get vitamin D into the diet.