Temperature shock - PLEASE HELP

Housing:
How many gallons is your tank? 5
Does it have a filter? Yes
Does it have a heater? No
What temperature is your tank? 73
Does your tank have an air stone or other type of aeration? No
Does your Betta have tank mates? What kind? Used to have a mystery snail, but I moved it to another tank

Food:
What food brand do you use? Tetra
Do you feed flakes or pellets? Pellets and Bloodworms
Freeze-dried or frozen? Freeze-dried
Live? 1 a week
How often do you feed your Betta? How much? 4 Pellets a day

Maintenance:
Before your Betta became ill how often did you perform water changes? Weekly
What percentage of water did you change? 75%
What additives do you use? Aq. Salt
What brand name of water conditioner? API

Symptoms and Treatment:
When did you first notice the symptoms? As soon as I put him in the tank after the water change
How has your Betta’s appearance changed? He is a crown tail and his fins seem clamped, his underbelly looks really really pale. And there are a few patches on his gills. His eyes look cloudy, and I believe it is because I switched back into his 5 gal before I let the sand settle down.
How has your Betta’s behavior changed? He is allowing himself to sink down to the bottom of the hospital tank, and is gasping violently. I lowered the water so he doesn't have to swim up as much. But every now and then he will swim back up for a giant gulp of air, and then sink down slowly. Then he will lay on his side, and dart back up to the surface to get some more air. He has also "shaked" around a couple of times, as if he is twitching.
Is your Betta still eating? No
Have you started treating your Betta? If so, how? I put him into the hospital tank, which is a 1 gallon tank. I added some aquarium salt and a bit of Furan-2 for the eye cloud... I lowered his water so it's about 50% full.
Does your Betta have any history of being ill? Not really, he had an inflamed gill once but I helped him get rid of that in a week.
How long have you owned your Betta? Next month marks a year.
Was he or she a rescue? Yes, he had fin rot when I bought him.
Was he or she ill when bought or given? He had fin rot.


I feel like a terrible betta owner. I switched him back to his tank after a water change and I didn't realize that the water was way too hot. Since I don't have a heater, he usually has 73-74 degrees. But when I changed his water, the sand was still mixed in the water and the water was maybe 5 degrees warmer. I don't know what to do, but I really don't want him to die. Please help me try to help him. He losing color under his belly and under his head. He is still gasping for air. Please give me some advice, I'll be really thankful.