I've had this fish almost 2 weeks. It was added with a Mickey mouse platy. They were all fine at first. Even my female betta left everyone alone for the most part (she would charge a platy over food, never touched the Molly).

It all started 2 nights ago. My molly would get near the surface and splash it's tail. I figured it was hungry, so I dropped a pinch in and it kept doing it.

This morning the molly is chasing my platys, getting right up beside them and flailing the top fin and smacking them with it's tail :/. It was sporadic but it's getting worse. It doesn't bite or nip though....

What the heck is this fish doing? As soon as I get the 55 set up it's moving out to another tank but I'm still a week or so from being done with it.

Hmmm Im not sure, but I know when I first got 2 of my female guppies (I know not a mollie) one female (smaller one) was kind of "annoying" the other one (larger one) and the larger one would "smack" the smaller to kinda tell her to leave her alone. Since then I have got a couple more female guppies and put them in my 29g tank and there has been no smacking around that I have seen. You may want to get another female molly to keep it company. JMO

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Tank: 29g:guppies, balloon mollies, swordtail, 2 mollies
Hex: opaline gourami
10g 1F. betta

Fish are like people, they all have different personalities. I have just Mollies in one of my tanks and it really is amazing how they all act different. He is probably a male, just trying to be the alpha male.

I don't really know if your molly feels comfortable living with a platy. In taking care of mollies, you have to know that they act strange when the water is in poor condition. The temperature should be checked as well. Some experts require to put some marine salt water mix to make it more comfortable for the mollies. You have to discover also if mollies live rightfully and comfortably with platys, because I ain't sure about that. Goodluck

Mollies and Platies are both community fish.

I believe all livebearers live fine together. I have balloon mollies, and guppies living together quite fine. Even on rare occasion a molly and guppy can breed together, very rare, but it has happened. So I don't think its a compatibility issue. Just watch for nipping, if you need to get a breeding net and put it in "time out" for about a week and see if the behavior changes.

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Tank: 29g:guppies, balloon mollies, swordtail, 2 mollies
Hex: opaline gourami
10g 1F. betta

Thank you. Temp is 78 as always. Parameters are good 0-0-10. Well aerated with an air stone and filter. All other fish and inverts are fine. I can't add salt because of my corys but I can set it up a salt bath. They aren't nipping, just the Molly smacking the platys. I'm going to try to get the tank done sooner and get him moved.

I had the same problem with my Dalmation Mollies until I moved them into my 60g. I had them in a 10g community tank that was overstocked. Once I moved them, no problem. I also can not add salt due to Cory cats in the 60 but the Mollies seem to be doing fine in the soft, salt free water in the 60g with the Rainbows.

A buddy told me the other day that he thought it was really just hard water as much as salt that made Mollies do well. I do not know that for sure, just what he said.

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60g tall, all artificial plants, Rainbow tank; 55g for African Cichlids. 20g long for fry; 20g hex, 2 black Angels at my desk; 10g planted, with 50 Molly fry; 10 gal, 2x 13w cfl lightning, planted; 2g hex for "Buddy the Betta"; Wanting a 200g +, MTS getting bad.

At least two mollies i had in the past showed this behavior. I think there could be one of the two reasons:

1. Just delivered fry and hence feeling the empty stomach
2. Fish is sick. I guess it will die shortly. Troublesome atmosphere in the tank could be the reason.