Help-Platys Dying

[Fraoch]

I have had platys in my tank for a year or two and last added 3 more about 3 monthes ago.
the problem is that the last 2/3 weeks, 2 of my platys have died and another cant seem to swim and is very close to dying aswell, as far as i can tell.
She is floating on the top and not able to swim(just being dragged around by the current). Her fins look split but its hard to tell as they are clamped, apart from that i cant see anything else unusual.
The last 2 platys to die, didnt show any signs before hand and simple dissapeared for a week or so before their bodies floated to the surface.

Ph-7.6
ammonia- 0
nitrite-0

EDIT; here is a photo(not very good )
i have moved her to a Qt tank and i also noticed that she has what looks like 3 little air bubbles stuck to one of her fins. Im not sure if they are air bubbles or not but they dont seem to be coming off.

 

[Fraoch]

i have found another platy with the same signs, except he can swim slightly better than the other ill one but he sits on the bottom instead of floating. i moved him to the qt tank aswell.

 

[Littlemonkey]

It sounds like your new platy's have bought something, (whitespot purhaps)with them wilder will hopefully confirm. The bubbles, do they look like little grains of sand on the fins?

 

[Fraoch]

It sounds like your new platy's have bought something, (whitespot purhaps)with them wilder will hopefully confirm. The bubbles, do they look like little grains of sand on the fins?

no, i think they were just air bubbles because they have gone now.
the first platy to die, when i found it had patches of white all over its body, just like the colour had drained from its scales. i dont know if thats just what happens when they die or what and it also had a split tail.

 

[Littlemonkey]

Keep up water changes in the QT tank, it's amazing sometimes the difference this can make, keep us informed please.