problem
 

iv had 2 platies now for a few months, they have been fine with everyone in the tank until yesterday.................
i got a beautiful new siamese fighting fish and his lady friend.
after all of about 10 mins the male fighter had most of his tale eaten!!!
now the guy i brought the fish off yesterday sai they should be fine and we thought so too as the platies (1 male, 1 female) had been so chilled out.
iv got the fighter in a breeding tank till he picks up abit, dont wan him in there to long or he'll get stress. any ideas whats going on???

Did you actually see the platties nipping him? Platties by nature aren't nippy, I'd be more suspicious of the female fighter.

yeah we did see him and her going for the fighter.
as i say i have the male fighter in the breeding tank and the moment and the platties are swminning around it almost waiting for him to come back out.
i dont wannt keep the fighter in ther for to long as its just gunna stress him and and most prob kill him that way.

The only thing I can think of then is that the platties, likemany "platties" sold in fish stores are actually platty/swordtail hybrids, and in this case they could be a bit agressive. If you don't have another tank then unfortunately, I think you'll have to return either the platties, or the fighter I'm afraid.

Bettas are sensitive to melafix, so if you're using that to treat his sore tail, use 1/5 the normal dose.

Sorry to hear it's not working out.