Good to hear he's doing better! It is still a good idea to use some tap water in with the spring water--spring water varies widely and is mineral deficient. It might be cheaper in the long run to use reverese osmosis water from a pet store, and then add in a product like Kent's RO Right or Seachem's Equilibrium.
Bettas don't tend to be themselves when you first take them home, it's normal for them to be stressy and not want to eat. You should definitely look into getting a good liquid master test kit (not the strips, they're inaccurate and expensive!) so that you can cycle the tank in the future. If you haven't already, you should research the Nitrogen cycle and how to cycle a tank fishlessly.
Good luck with Monet!