To flush LECA medium with tap water or reverse osmosis water?

[OrchideeNormus]

I have all of my orchids in self watering pots and the majority I use LECA as medium. I water using fertilized reverse osmosis water. I also flush all the LECA pots with tap water once a month. I recently ordered a PH and TDS meter since I'm focusing on trying to learn more about how to properly feed my orchids. Of course I wanted to check out my new equipment and started out by testing my tap water. I am on a public well so I didn't expect perfect water. I was a little shocked to see that the TDS meter read 647. I'm still trying to understand what this meter even tells me. I think it's basically telling me how many impurities are in the water? Should I continue to use tap water to flush my pots or is it ultimately going to be more damaging than helpful? Please help!