Whats this on this Plecos' fin?

[Pyramid_Party]

I noticed about a week ago some whiteness on the front of the plecos dorsal fin. Tonight I finally got a real good look at it. And at the base where it attaches at the plecos body, it appears to have a small bump. It also looks a little crusty at the base and its all white in color. I dont know what it is. I plan to do a water change either tomorrow or next day. Tank is always clean and crystal clear. Water parameters are always Ammonia 0, NitrIte 0, and NitrAte around 30. Temp is at 80 right now. Was at 78 then I raised it to 80. Heres some pics:

 

[Lupin]

Looks like your pleco is suffering from bacterial infection. Isolate the fish and treat with Maracyn.

 

[Pyramid_Party]

Well I got some Maracyn. The water is clean and everything so I dont know how this happened. Im thinking its secondary infection. What I think is the Pleco had an injury/open wound from a fight and it got this as a secondary infection. If water is clean and all, is bacterial infections possible? Think I should just treat the whole tank to maybe eliminate any bad bacterias in the water? Is this stuff plant safe?