If you want to adjust diet to identify an allergen, there are two approaches.
Remove the thing you suspect (in this case, fish) and wait a certain amount of time. (Wikipedia says two weeks to two months.) If the reaction clears up and doesn't come back, the thing you removed was the allergen. If it does not clear up, or clears up but then recurs, something else the person is still eating is the allergen. Choose a different thing to eliminate and start the process again. (Some people now restore the first food since it's been cleared of suspicion, others don't in case multiple allergies are in effect.)
Remove everything that might be suspect (Say, everything the person ate that day, or in extreme cases, everything except one or two foods eg rice and lamb.) Wait until symptoms subside, then add foods back one at a time. If a reaction occurs with something, never eat it again.
The first strategy can take a very long time if you don't know what the allergen is. It also has the issue of not being sure whether the reaction has just not died down yet, or is still happening because an allergen is still being eaten. The second is dangerous because the diet may be missing things you can't go without for even a few days, never mind for the weeks some reactions take to die down. You need the support of a doctor or nutritionist to consider it.
There are quite a lot of studies in the reference section of that Wikipedia article that you may find useful.