Extremely Thin Micky Mouse Platy

[peppermint]

Hi there. I'm kind of a fish newbie even though I've kept fish on and off through my life. So I've had my current 10-gallon tank for about four months. I have two guppies, a red wag platy, and two mickey mouse platys (orange and white). The problem is the white Mickey... when I first brought them all home, he/she was very healthy, eats well, active, and even a bit of a bully at times. But starting from about a month ago, he started becoming very thin. He still grabs food hungrily, but he does so in a seemingly blind and stupid way and ends up not getting much at all so he stays very thin. His fins are clamped. He's still pretty active and looks "happy" -- but kind of sways a little because he's so skinny now. If this keeps up, he will die soon and I want to save him because I've grown quite attached to this little guy.

White mickey

White mickey looked healthy and had a belly like my Orange mickey before this all happened!

All the other fish are healthy and active. Anyone know what can make him fat again?