answer:If it’s a big-finned, flashy kinda fish that looks like this (but could be some other color, too, like red or almost purple), it’s a boy. If it’s a small-finned, less vivid color that looks something like this, it’s a girl. :) The males are the ones that people usually buy, all pretty and lonely, in those little cups of water. You sort of have to go out of your way to buy a female.