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

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Back to advising OrchideeNormus.

With high-mineral water, your growing method results in more minerals concentrating at the medium surface, due to evaporation, than you would have with pour-through watering. I would use R/O water most of the time for self-watering pots. Once minerals form on the surface (efflorescence), they won't dissolve back into the water to any practical degree, so this would not be removed by periodic flushing with pure water.

You can mix RO with well water to decrease the mineral concentration, but it helps to know exactly what you're starting with. See how much it would cost you to have a water quality analysis done on your tap water. It's for your safety, as well. Find out concentrations of total dissolved solids, nitrates, sodium, phosphate, boron and metals. Find the hardness.

Your water company might have their annual water quality report online. I think in the US they are required to provide it, though it is true some small rural water providers don't always do the best job.