OK so to determine your fish sex/gender... Just look at bottom of the fish a ways infront of the back fin. And you will see that there can be 2 things. For a female fish you will see that it looks like a fin. Its a triangles shape and that's how its a female. For a male you look in the same place and instead of a triangle you will see more of a stick. Only its just how you see its male. Remember if you put a male and female together they will eventually mate mostly as the same fish.
And watch out for the fish that reproduce asexually. Because they don't need a mate to reproduce. And it can happen either gender when asexual. Some types of fish eat their babies and only few survive so watch out and have places to hide for them but not all fish of course. Yours without a doubt male its just natural chasing around
