HELP! guppy keeps getting black patches

My guppy keeps getting black patches, they are near his pectoral fins, and a little on them. He is acting normal except for this. I got him 3-4 months ago; he has been doing this for the past 2 weeks. The other fish in the tank are fine, including my other guppy.

 

Pics?

 

I can't upload any but they are like little patches; there not patterns and its mostly on one side

 

Water parameters?

 

Everything is fine(ammonia, nitrite, nitrate, ph, all that stuff). Theres some salt in the tank too