k9, I feel your pain. My male has had allergies his whole life and just a few weeks ago I bit the dust and had an allergy panel done to find out once and for all what he is allergic to.
It sure seems as if fish is the allergen. Most food allergies, a dog will have an allergic reaction to almost instantly after ingesting, so in your case it makes sense its the fish. They certainly can be allergic to fish, so it's a possibility.
It's odd he is not allergic to poultry or beef... most dogs develop food allergies to an ingredient that is widely used in pet food and that they consume most of their life (i.e. chicken/beef).
I wonder if he has more allergies? Is it possible he has environmental allergies as well? When I had Furby's allergy panel done he is allergic to beef, chicken and rice (which is odd) but is allergic to another ten or so environmental allergens (ragweed, fescue, timothy hay etc.) I'm not sure how common it is for a dog to be allergic to just ONE allergen... but I'm not really sure. My guy is allergic to about 13 different things (food/enviro combined) but maybe he is just a freak of nature LOL.
I think some members here use flaxseed as a source of omega's... I will look into it and get back to you.
My babies: Sassy - Maltese X (9), Furby - Shihtzu X (7), Brynn - Boxer (3), Diesel - Boxer (1)
"Many of the Earth's habitats, animals, plants, insects, and even micro-organisms that we know as rare may not be known at all by future generations. We have the capability, and the responsibility. We must act before it is too late." - Dalai Lama