Rotting fin

[Perthegallon]

Hello so my fish looks like this today and I don’t know what it could be since a new fish hasn’t been added in more then 90 days and they are all qt before and I even treated for this at one time to make sure rotting fish disease never got in my tank could this be the cause of something else

 

[Perthegallon]

Also he is the alpha so none of the tank mates ever pick on him

 

[Perthegallon]

Anyone?