I bought a beautiful blue crowntail fighting fish about 4 days ago. He was fine until last night when he seemed very lethargic and wouldn't eat. Overnight one of his eyes became very swollen and he can't swim very well. It became even more swollen during the day and I'm worried about how fast he seems to be deteriorating. He's just floating around at the top of the tank, not eating and not moving very much. He can't even float properly, he's often floating on the side. He's in a 20 litre tank (about 5.5 gallons) with a filter but I turned it off so I don't stress him out. The heater I had didn't work, but I thought he'd be fine as it's summer, but we're having a few cold days. His breathing, colour and fins seem fine but he's acting very sick. His eye had a whitish look to it so I added some fungal treatment to the water in hopes that it may help. The water is treated with dechlorinator and aquarium conditioning salts. I changed 50% of the water yesterday and 50% today. I washed his decorations with hot water today as I was worried they may have caused some infection (they were moved there from my other fish tank). Tonight's going to be a cold night, and he's barely moving as it is. Is there any chance that he'll make it through the night, and what else can I do to help him? (I can't get another heater until tomorrow).
He's swimming around a little more now, and I can see him better. His other eye is also swollen but not as much as the first. The scales on the top of his head look raised but the scales on his body are smooth.