I have just returned my platies to the main tank after quarrantining for 'bout two weeks. They fell ill with white spot so I treated them and they have been well for quite a while so I put them back today. They seem really happy and I just hope they don't get whitespot again. Thanks for everybody's advise.
Thanks but unfortunately they've got it again. This time I've treated the whole fish tank. It's really wierd because none of my other fish are carrying the parasite yet the platies still get it. I'm starting to believe the side of the argument that white spot is present in most tanks but the fish don't normaally get it unless they are stressed. The annoying thing is I did everything I could to make sure that the platies return to the tank was not stressful and the water conditions were the best that they could be. Platies are meant to be easy to keep yet I don't seem to be able to keep them but my other fish are okay. Any way they are already starting to show signs of being on the mend so hopefully this story will have a happy ending!