Aggressive Platy?

[ButWhyIsTheGinGone]

Hi all! Recently bought a beautiful blue platy from Pets at Home (I know!). I'd never seen a blue one, plus it was one of those with the Micky Mouse head on its tail, so was really pleased. I've put it in a tropical tank with 3 Black Phantom tetras, a small orange/red platy with black tails and fins, and 2 peppered corys.
Well, the bloody thing's an animal! It spent 10 minutes cowering away, then made a bee line for the other Platy! It chases him round the tank all the time - I often see the poor thing hiding behind the filter.
Tonight I watched her sneak up next to the orange platy then go "BANG" into the side of it... why's it doing this? The aggressive one is a pregnant (was preg when I got her) female, and the orange one male. Should I split them? Orange one isn't looking ragged/injured but I feel sorry for it!

Just like mollies, sometimes platys can be bastards. How big is your tank? The fish you have should be kept in groups of 6+ partly because they feel more secure that way and partly because any aggression is more spread out. But you shouldn't overstock so depending on the size of your tank you might need to trade some fish.

Did you quarantine the new platy?