Overly agressive male platy

[Sasquatch]

We're having problems recently with our male platy. He's gotten really aggressive, particularly towards one female.

This started since we've started cutting back on the food to reduce waste buildup in the tank. He seems to be focussing on one particular female, oddly enough the largest one in the tank. We're getting worried though because she is really stressed. With all the chasing around she seems to have injured herself (there's a little mark on her chin) and her slime coat seem to have been rubbed off. He chases her around a lot, bites her and is generally making a pain of himself.

We'd thought of getting something to isolate him, one of those birthing enclosures used for livebearers and I'll add some product to aid the slime coat today.

But is there any way to calm him down? Will it happen on it's own? We can't start feeding more 'cause our tank can't handle the load (we were getting really high nitrates and always trace levels of ammonia and nitrites) and we'd hate to take him back. He's the only male we've got and he's really nice and colorful.

Any suggestions?

[Lady Hobbs]

I'm afraid that's what you get when you have the two sexes together. I ran into the same thing with Mollies and they were such a pain I'd never have them again. I have only two platies, same sex, and they are always together and make great tankmates. If I dropped a female in the tank, all that would end.

[jweintraub]

I've got 2 males and several females in my tank to spread the aggression around evenly. What is your M/F ratio?

[Sasquatch]

We have one male and 4 females.

That's why the aggression surprises me. If I have two males/3females, I'd expect aggression among the males.

What's worrying me is that he seems to focus on one in particular and goes out of his way to harass her.

[jweintraub]

You mentioned that she was the largest of the females in your first post, have any of the other females given birth, if not it could be that she's the only mature female you've got?

[Sasquatch]

Actually, the two smaller females seem to be pregnant. We know that one has given birth and we think the other is pregnant, we can kinda see a dark spot behind her abdomen. (Her coloration is really good, so it's kinda hard to see past the scales/skin)

It's the two largest females, the two that were in the tank since the start, that aren't pregnant. The male had chased around the other one, but never got past "first base".:14: Until now, he has largely ignored the other female.