yucatan sailfin mollies

Question
i recently purchased 4 of the above, 2 gold and 2 silver. I know very little about their activities as you may find in my question !! after about 3 days, I have noticed that one of the silver mollies will not leave one of the golden's alone. its constantly chasing it round the tank trying what appears to bite/grab the tiny fin underneath the belly of the golden molly.
I was under the impression that these fish were docile and as they were previousy in the same tank there would not be any problems, its a 4 foot tank with a few plants and a couple of firemouth and pleco's in it. is this a sign of agression of possibly a mating sign ? i was not advised of the sex of these fish. please put my mind at rest !! regards.

Answer
Hi Andy,

Mollies will definitely mate with each other given the chance. They won't stay away from each other just because they are of a different color strain. The way to sex livebearers is below:
http://www.fishforever.co.uk/livebearers.html

Mollies are a bit harder to sex than platies or swordtails but it can still be done, look for that gonopodium in males. The firemouths will probably eat your molly fry, this is a natural form of population control. Certainly you can try to raise them, but make sure you have a place (or a willing LFS) to house your molly fry.

Mollies do best in hard, alkaline water with a diet high in greens, as they are naturally vegetarian. Here's more:
http://www.wetwebmedia.com/FWSubWebIndex/mollies.htm

I hope that helps, take care!
Nicole