Tiger barb infection

[FeldmansFish]

One of my tiger barbs, the same one who had a parasite like 2 weeks ago looks as if someone took a piece of sand paper to him. Checked tank water and all params are good. Tank is 30g with a bushy nose Pleco, 3 upside down cats, and 5 tiger barbs. How can I help him?

 

[FeldmansFish]

Sorry about the pic quality he's the one in the center of the picture.