Hello, I really need some fast help. I went into Walmart two days ago and saw one lone veil tail male betta surrounded by about five dead ones, and just couldn't leave him there. I've owned bettas before, and knew he was in pretty bad shape. His fins were torn and ragged, and his tail was just shreds. I decided to try my luck saving him. I brought him home and set him up in a one gallon tank, just until he got better, to try to help him. I treated the water with stress coat and settled him in. He's gotten progressively worse. His body has turned grayish, and the last time I went to check on him, he was just floating at the top, and I thought he was dead, but his gills are moving and he swam a little. I treated him with betta revive, and now he's sitting on the bottom of the tank on his side, still breathing but no other movement. He's not eating, and hasn't been interested in eating since I brought him home. I'm not sure what else to do or if he's even save-able at this point. Any help or advice is greatly appreciated.
As for tank conditions:
Size: As stated, a one-gallon until he's better
Temperature: 78 degrees at last check
Tank has a small air stone, no filter.
It is not heated.
No tank mates.
Food: I tried to feed him a pelleted food, and some blood worms, but he had no interest.
I've been changing water daily, a 100% change. Stress coat is added.
I listed all the ones I've seen so far up in my first paragraph.
Just noticed some fuzzy-looking white stuff at the base of his tail. Help would be greatly appreciated, as it seems like he might have fin rot and a fungal infection, and advice on the best way to possibly save the little guy would help greatly.