Sort of Looks like a bacterial infection to me, kinda hard to tell. Sometimes bacterial/fungal infections follow damage to scales by parasites.
Im not sure what otos are, so are they catfish ? sensitive to ick meds?
Acriflavin/victoria green sold at walmart might work if its a parasite.
If its a bacterial infection it might not be worth the time effort and esp. money on antibiotics to treat him.
Do you have any tank and stuff to go with it to use as a hospital tank?