Collect naturally dried and fallen from the tree Oak leaves...For medication-I use 20 crushed/gal in a 1gal jug of dechlorinated water-let them steep naturally for at least 30 min before use.
The longer they steep-the more tannins released-the darker the water-the more the Betta will like it.
For a tonic-I use a couple to 3-4 whole leaves tossed in the tank to release slowly and remove as needed-I leave mine long term to break down-but, I have natural systems and want the decomp.
You don't want to heat the leaf/tannin over 110F since that with kill all the good bacteria.
The Oak leaf and IAL tannins-both contain antibacterial/fungal properties, help to ease stress and can toughen scales/fins-As the tannins and leaf bits start to decomp they produce CO2 that can drive the pH down naturally-but this happens over time and why you want to use premixed tannin water for water changes-so not to cause any chemistry shock issues-to be safe when you use it as medication.
I have well water too and I don't add anything to it for use with my fish, however, not all well water is created equal-plus it can change with the season and weather-or at least mine does....still have never had any issues with it in regards to fish keeping.