Hi all, new to the forum and only had an aquarium for a few months.

Does anyone know what may be wrong with this female Platy. She gave birth to fry a few weeks ago, 7 survived and remained in the tank with the 5 adult platys, 6 white cloud minows and 6 Gold Minows in a 125L temperate tank kept at 22*C...Over the past few days this platy has this sort of white fluffy stuff growing on her, she will not swim and just sinks. She is now in a small 28l tank on her own with a dose of Melafix anti biotics.

New to this, learning as I go. Any help or advice would be appreciated.

Thanks,
Matt.

Very sorry to read of this and see this pic.

I'm not familiar with platies but I know they are better suited to and should be kept in hard water, so that might be a factor. These might be helpful so that you can ensure that you're giving them the environment they require:
http://www.fishkeeping.co.uk/modules/ ... sheet.php?caresheetID=202
http://www.seriouslyfish.com/species/search/platy

I have read recently of mollies sometimes dying shortly after giving birth, but I don't know whether this is common among platies.

Your platy doesn't sound too good. Melafix can be helpful although it's quite mild. I would try to get some eSHa 2000 (treats fungus, bacterial infections, finrot), and, if the Melafix hasn't helped her after a few days, consider dosing with the eSHa 2000 instead - but err on the side of under-dosing given how unwell she is.

Hope that helps and best of luck!