Epsom salt treatment would be 1 tsp a gallon. To help your Betta you really need to get a heater, it lowers their immune system and will cause more problems. Bettas come from Thialand, which means the water temperature is tropical. So a heater is essential.
Hope your fish gets better!
Oh and next time please don't post multiple times, it just confuses people.
15 Gallon NPT
1g no tech bowl in the making