fin rot -dark or discolored edges that are ''smudged'',bloody tips, getting shorter, falling apart, large part of the tail just drop off, ragged edges, fins have been looking frayed.
velvet -Velvet is hard to spot, but can be best spotted with a flashlight. Shine the light on the betta’s body: if it looks like it is covered with a fine gold or rust mist, then it has velvet. A betta with velvet will act sick, so look for clamped fins, scratching against rocks/gravel/tank, loss of appetite, loss of color etc...
Ich -It is easy to diagnose a betta with external parasites: It will dart and scratch itself against anything it can find, such as gravel, rocks, heaters, tank walls, etc… You may or may not actually be able to see the parasites themselves. Look instead for a behavioral change in your betta. If it looks like it is on speed or acid, then it has parasites!! :)))) Betta with ich also looks like sprinkled with salt or has white dots that will spread pretty fast.
velvet and ich is external parasites so you will treat the same. Since you noticed some very fine gold he might has velvet.
I am going to give you the link to follow instruction . Check out my and LittleBlueFishlest instructions. Check post #6 and post #10
Is he acting any differently like trying to scratch himself on objects, darting?
Also for 2 you need to do 1-50 and 1-100% water change a week. A lot of people will even recommend 2-50 and 1-100% a week..Unfortunately the water changes that you been doing is not enough at all. I would think that why he got sick. But hopefully you can treat him. It good that you dimmed the light since those exteranal parasites due to a photosynthetic process , meaning it will get worse with light.
Also you said that you just add some salt . Some salt will not help make sure you follow the instructions on that link i gave you . It very important to follow right dosage and right duration.
And give us update because some times you also will need medications to use , it all depends i guess on severity and individual fish.
If you can treat fish in his main tank or you can quarantine him. I would take everything out from his tank and rinse it with hot water and just let it sit outside in the sun , make sure it completely dry out -it will kill any parasites. Just let it completely dry out. Put all stones separate so they don't touch each other. Do you have live plants?
Also i would do a few 2-3 50% water changes first before you will do 100% water changes. And do them with about 2-3 hrs apart. The reason why i recommending it because you don't want to shock him with drastic water chemistry difference.
Aquarium salt also will help with fin rot, just don't stop even if you think he is better. You need to do full course of it.