Possible pleco fungal/bacterial infection?

[Tacocat]

I have just finished quarantining my plecos in a box for 3 weeks, and when I put them into the tank I saw one of them had a white fluffy spot on the base of his dorsal fin. I fear infection but he has been doing really well other than the stress of transfer.

Diagnosis?

 

[Colin_T]

Possible physical damage (small wound) that should heal up by itself if the tank is clean and it doesn't get infected. You can add some salt to reduce the chance of infection (1 heaped tablespoon of rock salt of every 20 litres/ 5 gallons of water). You can also try doing a 75% water change and gravel cleaning the substrate every day for a week. This will dilute any pathogens in the water and reduce the risk of infection.

If the area gets worse or goes red, it will need some a broad spectrum medication that treats bacteria and fungus.