Sounds like carnibest Nash? It's great food, no need to add oils though!! There's already cold-pressed sunflower and linseed oil like you mentioned and translated from the site: "The salmon oil is not intended to supplement carnibest, because all variants are equipped with Omega-3 fatty acids already. In principle it is not necessary to give salmon oil over it."

It doesn't really matter whether his brothers/sister or sire/dam do good on the same food or whether he been doing good on it before. Each individual dog can have or develop an allergy or protein intolerance for certain type of animal.

There's no way someone will tell you what's the problem. Best way to find out is to rule out all possible problems It could be a protein intolerance for beef or chicken, it could be the rice, one of the oils or one of the many external factors.. house mite or grass allergy??, ect. Reddish paws could indicate its a food allergy. I can recommend "haaksbarf" if you wanna try an elimination diet. nothing besides the animals itself and 15 % vegetables is added. They have horse, rabbit, goat, duck, salmon, lam. But id look for other options too, a blood test can point out some external stuff.

It can be a pain the ass to find out what's the problem cause it can take a few weeks to see improvements. So be patient there

No problems with the old food?