Molly harassing platy

[jlinsey]

We have had our Eclipse 6 set up since January this year, and, after losing our first two fish in the first few days, now have a platy, a silver tipped tetra, and 5 mollies that were born right at the start. Some things that have happened recently seem to be related to the mollies getting bigger. One of the mollies is clearly the biggest and over the last month or so has started to chase and nip at the other mollies and the platy. Two other mollies are just slightly smaller than him, and the two others are MUCH tinier, although they do eat and appear healthy. Is it normal for babies the same age to range widely in size like that? Is is related to gender?
Two weeks ago, we had two additional small mollies, but they disappeared overnight without a trace...I assume they were eaten by the biggest molly. (Five months before that, another baby molly died, but that time I did find remains.) This morning I found the platy wedged vertically behind the heater and thought he was dead, but maybe he was just attempting to hide and got stuck because he swam out when I moved the heater. Now I have done the water change that was due to be done today anyway (usual is 50%PWC every ten days) and he is swimming normally. I also changed the filter. I am hoping that those two things will minimize the big molly's bad behavior, but am not sure whether that will be enough. I have no idea of the gender of the other mollies...perhaps there are not enough females for the big one that I am assuming is a male?
Or maybe our tank is just too small to keep mollies happy, and we should think of finding a new home for the bigger ones? Right now the biggest molly is just slightly smaller than the platy.
I just tested the water...nitrite 0, nitrate 10, ammonia 0.
TIA

 

[kostasonia]

In an older tank, one of my platys used to follow the rest, the molys where hiding to avoid him and yes..some mollys were bigger than the rest...One of them was hudge. When the "bad" platy died, a zebra started following everyone in the tank.

How big is your tank? Do you have plants? A fish shouldn´t need to hide behind the filter if there are plants, rocks etc...

 

[rich311k]

That is way to many fish for a 6 gallon tank. Fry do grow at different rates and become different size adults, just like people. You should be able to tell the sex of the fish by now. Females have a split anal fin, males have a modified long thin anal fin.

 

[jlinsey]

We have a plant, a rock and a castle, so there are places to hide.

Since I did the water change, the molly is still bothering the platy occasionally, but the platy is no longer hiding .

Thanks for the info on the M/F fin differences; now I know the biggest molly is a male and I think all the rest are females, although it's a little hard to tell on the smallest ones.
We never really intended to have this many fish...we just got the babies early on and wanted to try to raise some of them. I guess I will try to find a home for the three largest mollies, then we'd be down to four fish that would hopefully get along.