I have exchanged emails with Ron and he states he is open Tuesday through Friday, as usual. I’m sorry the OP thought otherwise. Perhaps the OP could retract his post, so that Ron does not lose customers.
I don't know about the best but Lab. Epsilon in Ajijic says they do water analysis. They are in the back of a small center immediately west of Biencom Real Estate/Marissa's Bakery. Open from 8.30-4.30. They also do economic Covid testing at 200 pesos, wait for the results.