Only One Male Platy But ...

There are a couple of possibilities.
He could just be objecting to a new fish of any sort, which will settle down in a few days as he gets used to the new female.
Or the new female could be a juvenile male that hasn't fully developed yet. We may not be able to tell the difference but the platies will. Keep an eye on the new fish for signs of it becoming male 


And also keep an eye that the male isn't damaging the new fish. If it is female things should settle down quickly.



Edit - another thought - is the male trying to mate with her? He will have already impregnated the first females, now he wants to make sure he does with the new one.

The triangular fin is female. But young males also have a triangular fin until puberty when it changes to a gonopodium. Platies in shops are usually mature, but there is always a possibility of a late developing male. (With swordtails, for example, it can take up to a year for a male to mature and is the basis for a lot of the stories about sex changing swordtails, but this would be unusual in platies).

I suppose your male could just be more interested in the new female. We cannot know how fish 'think'. When you say a quick jab, do you mean he is pushing at her with his head or with the side of his body? If it is the latter, I would say he's mating with her, having had endlers which mate the same way.

I would put it down to just a quirk of this fish, since he is not harming the females.


It's just livebearers that do this with the anal fin as they grow. That's platies, guppies, endlers, swordtails and mollies, plus a lot of rarer species you are unlikely to see in shops.
Newly born livebearer fry all look female. Endler fry are all colourless, males slowly develop colour as they grow while females stay colourless throughout their lives. Well, grey beige which is almost colourless  After a month or two, the anal fin of the male fry begins to change shape. The first ray elongates, then the rest shrinks until the fin has changed into a gonopodium. When I had endlers, I would separate out the male fry as soon as the first ray started to elongate.
I think platy fry are born with some colour since females are just as bright as males. But again, all the fry will look female at birth. As the males mature, the anal fin will change shape the same as for endlers.
Something for you to look for when you get fry