He developed problems after 3 years? But he was fine up until then?
How did we determine the fish? I had him off of all egg product (I forget what food we were on at that point) and he did really well, his ears were clear for a long time. Then I read here about giving their dogs salmon, I think, as a treat. I think it was salmon...anyway, I did, and both ears flared up almost immediately. That was enough for me!! We avoid all fish product now....
He was on NB potao and duck up until all the food recalls. I stuck with them through the recalls, but shortly after, he developed problems. Cost me a small fortune in vet bills, thinking he had Giardia, though tests came back negative. Finally just switched him cold turkey, poops cleared up in 2 days. Then last year, thought I would give them another chance. Tried the Sweet Potato and Venison, did a gradual switch. Within 2 days after being on the food 100%, mucus and blood in stools. I'm done with NB.
As for your Salmon treats, maybe it was something else in the treat? Where they made in China. From what I remember, one you were feeding EVO, which has some fish in it, but also had egg. I mentioned that some dogs have issues with egg. That's when you switched back to Purina.
Have you ever tried a food that did have fish, but no egg? Also, do you give any supplements such as joint supplements. Some of those contain soy products.