There's no easy way of finding out which fish is missing, I'm afraid.

My tip is to breed and sell the canary fire-arrow in one tank (i.e. make lots of money) and then buy new eggs and then use the unknown chemical on them until your missing fish is 'found'. It's an expensive but the quickest way of finding the remaining fish. After several doses of the unknown chemical, you may want to cross breed existing fish before dosing them with the chemical again. I find that the chemical seems to work in sequences so you need different fish after a while.