Black Dusting On My Gourami's Fin

Hi,

This evening I have noticed that my beloved gourami has a strange black dusting on his fin. He's seemed a little less playful over the last few days but hes still swimming around and behaving fairly normally.

I know the tank is overcrowded but the water stats are fine, I'm in the process of cycling a new tank to sort the overcrowding out. Like I say, even though there are theoretically too many fish this isnt showing up in the water stats

Have a look at the photo, any ideas?

Thanks for your help!

Paul

Can you post your water stats please as black patches on fins can be ammonia burns that are healing.
Any signs of flicking and rubbing.
How many gallons is the tank.
How many fish and which type.
What filter do you use.
What test kit are you using.

No flicking, no rubbing, ammonia 0 and has been for weeks, NO3 trace, NO2 0, PH7ish. Using a API ammonia test kit and test strips for other measurements

60l tank, 5 zebra danios, 6 neons, 5 five stripe barbs, gourami, sucking loach.

Its a Juwel tank, but currently treating whitespot so just got the blue filter sponge in there, along with the white pad.

Ok it is slightly overstocked, but neon tetra don't produce much waste.
Test strip cards are not accurate.
See how he goes, if no changes get back to the board.