Need help. Sick male betta

[smashedkittkate]

My male betta finni seems to have a sickness i cant diagnose, his scales have turned grayish around his head, gills and random spots on this body. My male betta developed fin rot and a fungal infection from the main tank because of a dead fish that i couldn't find. (Im still beating myself up about that :c) i seperated him and had him in the hospital tank with medicine for a little over two weeks now. This fins are growing back healthy and I'm pretty sure his fungal infections going away (it turned to a brownish black color now) but i still dont know whats wrong with him. He acts like a happy healthy betta, always a hungry little piggy, coming to the glass when he sees me.
Please tell me what I can do. I want to help him out.

 

[your bro joshi]

Put a pic of your Betta if possible ?

 

[smashedkittkate]

I was finally able to get him into a cup to take some pictures. I took pictures of him from both sides.

 

[your bro joshi]

Looks to me mouth rot fin rot tail rot. Treat him with tetra cycline from mardel lab. Do not put filters on after medication as it will absorb it. Try to give quality food and try aquarium salt