It is every single thing I feed period. No matter the brand, shrimp, fish fillets etc..... My sump is clean and does not need replacing. What about a UV sterilizer in case it's something in my tap water chemistry reacting with the foods?
Did you try to re-create the smell with tank water and pulverized flakes (outside the tank?)
Did you have someone else confirm that they smell the same smell as you do?
Must be frustrating.