What is this ?

[Decoder]

This BP has had this wound for 2 months and it is still there despite frequent water changes and antibiotic treatment. How can I cure this thing?

 

[guppy]

You can try swabbing the wound wit a Q-tip and either iodine or hydrogen peroxide solutions. Do this a couple times a day, wear wet surgical gloves when handling the fish. It is stressful to the fish so add a slime coat enhancer to the tank, maybe melafix or 1 tbsp salt/5 gallons.