fin rot

[Argenteus]

I have 4, 4-inch silver dollars in a 45 gallon and I noticed that some of them have some black edges on their tail fin that is slowly degenerating. It seems almost like fin rot, though usually it should be edged in white. What could be causing it? Is it salt?- I have about 1 tsp per gallon. I heard its good for most fish, so I add salt. I hope its not caused by carbon. I do about 10% water changes every week and all my water parameters are fine. (ammonia and nitrite are 0).

 

[Menagerie]

Could be bacterial or fungal. Salt and carbon should not have that affect on the fish.