Umm... I have about 40 parrot fishes in my tank and recently, I have noticed 2 of the fishes floating with its belly near the water surface. What's the cause of this illness?

There is no way for us to know. Have you had the water professionally tested?

Chances are they are going to die, and you should put them in a small container of their own, and take to a pet store. Many diseases are communicable. Do you see any little white spots?

Sounds like their swim bladder burst...

This is treatable if they haven't been allowed to float like this for too long. A swim bladder infection is similar to a UTI in humans.

Put them in a separate tank and crank the heat up to about 80-85, add salt and get antibiotics for them. Parrotfish, because they are mutt fish, have this problem quite often. It is a result of a dirty tank or over crouded tank. Parrots get pretty large, so I'm hoping that you don't have too many crammed into one place.