are my guppy sick?

hey all,

I brought yesterday guppy fish.and part of them have big belly (include pictures). I have aquarium with filter and air pump.are they sick?
i m a g e s h a c k . u s / p h o t o / m y - i m a g e s / 4 4 0 / p h o t o 0 6 5 9 . j p g
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