This appears bacterial and almost represents Brooklynella in appearance guessing gram-negative. You may then ask, how did my fish get this or "catch this". There are many possibilities and some are :
- Poor environmental conditions.I If conditions are poor enough, the bacteria can bloom and overrun even the healthiest of fish and this can be from the place of purchase.
- secondary infection resulting from having another disease such as an open sore left by an infection such as cryptocaryon. The fish has been cured of the parasite, but the resultant infection from bacteria entersthe fish's body
- Poor nutrition
- Injury, such as open wounds, cuts or scrapes
- harrasment from tankmates
- Stress
Using the proper type of antibiotic is of the utmost importance and in this case, Maracyn 2 which is mincycline hydrochlor based and is a gram-negative bacteria treatment, as well as sufficiently absorbed through the skin to treat internal infections.
Other options are neomycin, chloramphenicol, nitrofurazone Furan-2 products, skin absorbed kanamycin sulfate-based antibiotics such as Kanacyn/K-Mycin