Corydora with white fungus on fin

My juliI corydora has a white puff ball like fungus on one fin. Was small at first but seems to have gotten bigger almost covering the whole fin. He seems to be his happy hyper normal self, no signs of distress and his tank mates haven't showed any of the same symptoms. ( 1 julI cory, 4 adf's, betta & pleco) I barely spotted the same fungus on the lid of the tank while doing a recent water change and scrubbed it off. I have done a lot of research but can't seem to find the exact name of this fungus and what medication would be best for a cory?? I attached pics its on his right fin. Any advise would be grately appreciated!

 

Think it is a secundaiy fungal infection. Try Esha 2000. Substrate isn't very suitable to me and might have caused it.

 

What kind of subtrate do you suggest? I was having second thoughts about it as well thinking its too rough for them.

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Sand / fine gravel?

 

Could you pretty please do a quick test and report back with parameters for ammonia, nitrite, and nitrate? We can better assist you once we have those numbers.