Costia?

[cjsans]

I have a 125 gallon tank with Barbs, Guppies, Tetras, Botias, Glass Cats, and Gouramies. It's the Gouramies that seem to be affected by a skin condition that appears as a large discolored area or areas behind the gills on the back. From what I've read I'm thinking Costia. I will euthanize the 3 affected fish, (if I can catch the buggers), but what can I do about the cause? Will it affect my whole tank? I assume this is contagious and not just from poor water conditions, otherwise, why just the Gouramies? Is this specifically a Gourami disease? In all my years, I've not come across this before.

for your assist.

 

[HUKIT]

Costia is a single cell microscopic parasite that attacks the skin of fish. It is a flagellated protazoa. It destroys the skin, in mild cases the skin appears cloudy but in severe cases the skin is open and bleeding. This normally attacks fish that are stressed due to deteriorating water parameters.

Please post pictures prior to euthanizing the fish and current water parameters?