I've just brought a blue betta fish (my first fish, and betta fish) 3 weeks ago. He was very sluggish and just stay on the bottom and would not eat at first, but the second day he ate, and he got used to me in a couple of days, and is more active now. He's been making bubble nests every since the first water change too. He likes to rest on the bottom sometimes. About week ago I noticed grey spots on his fins, but it has been getting increasingly more. I'm not sure what it is. It reflects off as blue when viewed on the side. Could it be a sign of columnaris? The pectoral fin lines and outer overall shape are still there though, nothing split, so it doesn't seem to be fin rot?
I have been changing his water 100% every 6 days and for the last 2 water changes, been adding a bit of aquarium salt. Tank has no filter and unheated (hence the 100% change). The tank is small- more like a fish bowl (cube). I feed him pellets usually twice a day except once a week I'd feed him only one.
My fish was approximately 3 weeks old when I got him so he would be about a month and a half years old.
He seems to be normal otherwise. Eager to eat. Follows my finger.
I have attached pictures. Any comments/suggestions on what it is would be much appreciated!