Flush with tap water

[marcoeverard]

Hi I transferred all of my orchids over to Simi hydro last year and they seem to be doing pretty well including this coelogyne cristata that has never flowered before.all my water comes from collected rainwater and I was just wondering what the thoughts are about if I flush the pots out with tap water which is a little bit hard and then top up until the water flows out the bottom of the pots with the rainwater. at the moment I am filling the pots to the top with the rainwater and obviously this uses up a lot especially on the bigger pots. However I wondered if I fill up each pot with tap water and in run through a smaller amount of rainwater would that be harmful at all?