No experience asking a girl out in a fish store, however, I do with anywhere else.lol Cute chicks where I live rarely ever keep fish. They prefer dogs in general. Just ask her if she can go on a date with you and don't forget her phone number.
I used to all the time, but I was working there and would simply offer to "help them with their tank". I actually met my wife while maintaining a tank in her place of employment as well!