Is my fish sick or frightened?

[manky]

Just put in 15 juvenile male fish in a 135g tank fully cycled about a week ago.

Initially started with 10 and then added 5 more after few days. All the others are doing well, except one... The problem is, he just keeps of floating with his mouth consistently opening and closing. He floats near the top but will swim down to hide in the cave when needed. He does move when provoked or hides but he doesn't swim with the others and kinda just stays in the corner floating.

He does not seem bloated or anything, just kinda floats around, and sometimes floats vertically with his head down.

[deeda]

It could be stress related and the fish is being harassed by the others.

Which species fish do you have and which one is the one that is having problems?