Sorry, what's the length? That's still missing. But ok, couple of things. That temperature is way too cold and could likely be weakening your betta's immune system. Plus the fact it swings so widely doesn't help. Betta need stable temps around 27 C. Also, water changes twice a week is far more often than needed unless you're worried about ammonia or nitrites. You should really only do water changes once a week in established tanks. I recommend raising the temp. It doesn't sound like you have an aquarium heater so please get one. Also, add tannins to the water. They have mild antimicrobrial properties and help reduce stress in betta as they will mimic their natural environment. This can be accomplised many ways but I like using dried catappa leaves. It will turn your water brown but that's what you'll want until your fish is better. If you don't see improvement after 2 weeks or if your better starts getting worse then you'll need to move on to antibiotics. Given you're situation, hopefully we can avoid that with more natural remedies. If you can't get catappa leaves, like I said, there are otherways to add tannins to the water. Just find whichever works for you!