50 Gallon Tank White fuzzy piece in Cory's gills. Please help

[Jona8805]

Hi all,

I noticed one of my cory's is separated from the group (6 total), he hovers still in the flow, or goes up to the surface.

Upon close inspection i noticed a white fuzzy piece in his right gills. Any idea what this could be? It's not solid but rather fuzzy when viewing very close.
Gills seem reddened to..
Please see the pictures for reference.
Many thanks in advance for any help.

Tank parameters:

Tank size: 200L
tank age: 2 years
pH: stable 6.5
ammonia: 0
nitrite: 0
nitrate: 10-20ppm
kH: 6
gH: 6
tank temp: 23c

 

[Spinich]

Looks like a bacterial infection to me, possibly Columnaris?

 

[CindiL]

If it’s cottony I’d say it’s likely a true fungus vs bacterial infection (like columnaris). You will want to treat your whole tank. Seachem Paraguard has malachite green in it that can treat it.