It could be the oil used to fry it in as its used several times before they change it.
I used to eat fish and chips often years ago but now my stomach can't tolerate it. For several years now I find that when I have fish and chips I suffer with gut problems. I get bloating, excess acid and indigestion. In fact I can't seem to tolerate most takeaways these days. Tusday night I ordered a pizza, some nachos and bbq chicken wings. I left a lot of the pizza and chicken wings but ate all of the nachos. I suffered quite badly that night. I've got IBS anyway so I have to steer clear of most fast food takeaways. Kebabs have the same effect.