Food allergies cause an immediate or almost immediate reaction. Food intolerences are slower burn and reactions build over time.
If you haven't noticed immediate reactions after eating then it's more likely to be an intolerance and most intolerences are often (but not always) protein-based. You cannot medicate for food intolerances. Apoquel is used, primarily, for environmental allergies.
So, it may be that a fish based diet will help but 1 week is far too short a time to see any real difference if it was a food intolerance causing it. 4-6 weeks minimum. Food intolerances are about playing the long game.
As a way to prove/disprove the theory you can chose the novel protein/carb approach above or can get hypoallergenic foods that have hydrolysed proteins in them. These proteins have been broken down so that they no longer cause the same reation. These are veterinary based diets, not just 'normal' pet food that is sometimes labelled as hypoallergenic.
Whilst you may not want to use these diets indefinately, they can be very useful to rule in/out intolerences.
Eg. PRO PLAN Veterinary Diets Canine HA Hypoallergenic Dry Dog Food as a kibble based main food. Maybe coupled with Royal Canin Dog snack Educ Low Calorie treats.
Do not feed anything else but that for 6 weeks over which time you would ideally see an improvement.
If you do see an improvement you know you are dealing with a food intolerence. If you don't, you may look elsewhere for the cause.