New Betta Dying?!

Hello, all! It’s been awhile since I’ve been on here. I just got a new betta on Sunday and today he is already dying. The first couple days he swam around but was hiding a lot. Today, he cannot swim and is stuck at the bottom breathing very hard. I have no idea what happened, but I noticed his gills were blackish. What could have happened to him? I uploaded 4 photos of him that show his progression. Please help!!

 

What size tank? What filter? What parameters?

 

Also he has red spots all over his body which is new. It’s a 2.5 gallon TopFin tank with filter and heater. I haven’t taken parameters yet but will do soon!

 

If you haven't taken parameters, how do you know the tank is cycled? The red spots are likely ammonia burns. I'd do 50% water changes every day until the cycle is established. What sort of water do you use in your water changes? If it's tap, what do you use to treat it?

 

So I just noticed he has white stuff growing on his body. Fungus maybe? I’ve put prime and stability in the tank, as I did with previous Bettas and had no problem

 

So I just noticed he has white stuff growing on his body. Fungus maybe? I’ve put prime and stability in the tank, as I did with previous Bettas and had no problem

Also I use tap water, with the daily doses of prime and stability