Harlequin gasping for air and big white spot on body

[Icyfish]

Since yesterday one of my harlequins started gasping for air constantly and breathing fast when not at the surface. Later when i moved it to my quarantain tank i noticed a "big" white spot on one of the sides, which sticks a little bit out. In general this fish is also considerably smaller than my other one's and barely moves since lately, so something is definitely wrong. Could this white spot indicate Ich? The other 8 seem to do very well and have no signs of strange behaviour or spots. I hope the pictures are clear enough. Help is much appreciated!! Thanks a lot

 

[Salty&Onion]

Gasping usually indicates that there is something in the water that the fish doesn't like.
What are your water parameters?