I would so buy him off he were an HM...and if I hadn't already purchased about 8 Thai bettas in last 2 mos!!
Callistra: I currently have about 25 adult bettas and most are from Thailand! I have had no difficulty keeping them on my Oregon well water, which is, thankfully, free of contaminants and heavy metals, but is about medium hardness...nothing like hard water in Moscow, Idaho...but still fairly full of calcium! I count my family lucky because we don't have to drink(or bathe in) fluoridated POOL water like our nearest city residents have to, but I still have to use a pumice stone on my toilets on a regular basis.
Anyway, I have no problems acclimating my Thai bettas to the water...although 1: almost all my tanks are NPTs with a light spreading of peat which would make them a touch softe(keep in mind that I have plenty of breeding males that live in 1-3 homes that are bare bottom), and 2: I have not attempted to keep CTs which I understand live much better in softer water.