Kitten Diarrhea Due to Fish (from Wellness Core Canned)?

Kitten had diarrhea for the first time in the past 3 months. I suspect it's the Wellness Core Salmon Whitefish and Herring canned  food I offered him last night, which he ate about 40% of a 6 oz can mixed with the chicken and turkey formula that he has been having all the time. It's the first time he had fish and I've read it can cause many issues, not just diarrhea, for many reasons. I wonder if anyone here have encountered the same problem with fish. 

He pooped twice in the few hours afterwards - the first time with well formed stool, the second time diarrhea. 

I had a cat who couldn't tolerate any fish flavored foods.  It would cause her to throw up.  I'd stick to what you know doesn't cause the bowel upset.