Mystery Illness

Hi everyone!
My red veiltail betta Reggie has been ill the last two days and I'm stumped.
Thursday he was lying at the bottom of the tank all day and refused to eat, which he has NEVER done in the 10 months I've had him. Now he's keeping towards the top of the tank, gulping air every few seconds. I shined a flashlight on him and noticed some white wispy threads on his body - could be a fungal infection? Not sure if that explains the heavy breathing or refusal to eat....
I moved him to a QT and am dosing with 2tsp of aquarium salt per gallon, and I lowered the temperature to 75 degrees - I don't want to medicate if I'm not sure what illness he has...
Has anyone ever experienced this? All suggestions are welcome!

pictures of the fish?

what is the ammonia, nitrite, nitrate and pH of the tank water?

do a 75% water change and gravel clean the substrate every day for a week.
make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Hi!
I just tested the water--- Ammonia: 0.25, Nitrite: 0, Nitrate: 0, and the pH is 7.2.

I'm dosing with 2tsp of aquarium salt per gallon, but I ordered Kanaplex which is coming tomorrow so maybe that will make a difference. I've been doing water changes every day and adding stress coat.

(Also here's a picture. His fins grew back a bit strange after a nasty bout with fin rot when I first got him. It's impossible to see the white stuff growing on him unless I look super close... not sure if it's fungus or not? It's hard to see any inflammation because of his coloring. He's completely refusing to eat.)

Again, thanks for all your help! All suggestions welcome!

Can you post a photo of the whole tank please

Hi!

I don't have a pic of the original tank - I took it apart to clean when I realized something was wrong. The tank was pretty sparse- just him and a couple of plants. I was keeping up with regular water changes so I can't figure out how this happened. I've had him for 10 months, and this has never happened before.

Right now I have him in a tiny holding tank because it seemed like he was struggling to make it to the surface in the 3gal.

Should I worry about him not eating? I know bettas can live for a while without food but I'm a bit worried - I got him to eat 2 little pellets yesterday but nothing today.

Give him some floating plants, so he feels safe.

Great idea! Thanks!