why are my fish hanging at the top of the tank??!!

hi,
my platy and 1 or 2 of her babys are hanging at the top of the tank
seemingly gasping for air but i have only recently put a new airstone
in. my other fish are fine eating normally i am giving the tank a
routene clean today, but, i had some fancy guppys they started doing
this and then died, i want to save this lot because i was loosing 1 a
day with the guppys, i dont want to loose the platys because they are
my favourite fish and they are so friendly aswell

please help,

danny