+1.Seachems Prime might be worth getting to replace your current water conditioner. Prime actually turns any Ammonia and Nitrites into a less toxic form so may help there if there are any issues. And you can add that to the tank per drop gallon in between water changes.
I would say for your tank you need to do two 50% water changes a week. using a gravel siphon as that helps remove any waste in your gravel.
Per Hallyx (who advised me), 1 drop Prime per gallon per day. 2 drops per gallon in new water with each water change. I put the Prime in a dropper bottle, so much easier! API test kit highly recommended.
100 percent water changes might be stressful for betta. That is why 50 percent twice as often could be better. I make sure new water is up to tank temp before adding.