Sick Betta

[aqua-noo]

This is my first Betta bought him this past Sunday. I notice that he has a growth on it's doral fin. It is turning black like it is rotting. I have increased the temp to 30 degrees and added aquarium salt to the water after doing the routine water changes. He swims ok but his fin is bothering me and he seems very lethargic just lying at the bottom of the tank he won't eat even if I give him frozen blood worms. Not sure what else to do.

 

[WILDER]

When fish turn black it can be ammonia burns that are healing, but he dosn't sound right to me, it sounds like he has a bacterial infection so i would treat him for one, good luck.