Yes you can reuse your cooking oil after frying fish.
I reuse my cooking oil after frying fish and other foods many times and only change the cooking oil every few months to every 6 months depending on how much fish or other foods I fry.
I've fried potatoes and chicken in the same cooking oil that I fried the fish in before and it never has been a problem.
You might get a fish taste on the potatoes or chicken or any other foods you fry in the same oil after you fry the fish but for me I don't mind having the fish taste on the other foods.
I love fish so the taste is not bad for me but some people may not like it.
You don't have to use fresh cooking oil every time you cook something in it and can reuse the cooking oil many times before you need to change it out.