Help, all experts~
I think my white betta (male) is sick. Symptoms: 
1) I notice he lost his appetite,
2) Seems only breathing from air only.
3) Keep staying at the bottom of its tank. The only time he move is swim up to the surface to breath.

I then move him to a small transparent tank within a big tank of long-tail, colourful guppies. He don't seems interested with the guppies that swim outside his small tank. I move a pair of guppies into his tank, hours later, they are safe. (In fact, when I first bought him, he don't flare to its own kind too, except female... May be its his character thingy? Not a fighter, but a hamsap fish. I think he love SM and indirectly killed my white female... )

He hasn't been eating, but the stomach looks normally full...
Anyone know what happen to him?

Guys. Need help. My betta fish is not doing well.. he have not eaten much for the last 2 week. In fac e have not eaten at all. Also he look very weak and not as active anymore.

The only thing I did was to change the water.  Other than  that nothing extraordinary happened.