I have similar issues, all the tap water around me has quite high - my last house it was well over the legal limit of 50ppm but my new house seems to be between 20 and 30ppm. I've used a nitrate filter in the past which is something you can pre filter your water with when you do the water change, you have to run the water through it at a trickle to remove it so you need to plan accordingly. I used the Pozzani brand ones.
An other option would be to add some fast growing plants to your tank something like Pennywort or Hygrophila 'Siamensis 53B' really soak up nutrients. The Pennywort can be floating or you can plant as a stem like plant. Other floating plants would help as well and your Betta will like some of the long roots they send down
Wills