First of all, there is no link between omega-3 fatty acids and reactions to fish as for the capsules the fatty acids are separated from the fish proteins.
Secondly, you should be aware that fish proteins are found in a lot of processed foods, where you wouldn’t expect them, for example in Caesar salad and Caesar salad dressing, Worcestershire sauce, bouillabaisse, imitation or artificial fish, or shellfish (surimi), meatloaf, barbecue sauce, or caponata, a Sicilian aubergine relish.
Thirdly, it might be a matter of cross-reactivity which between fish is quite complex and has not yet, to our knowledge, been conclusively defined. It depends on what specific protein(s) your antibodies are directed towards. Many patients can eat certain fish, whereas they react (sometimes severely) to others. There are no definitive data regarding cross-reactivity that we can apply to fish in order to guide patients in their choices. However, the fact that they are in separate families does not rule out potential cross-reactivity.