Hi, there is no right or wrong answer, some people use RO some tap some bottled, when flushing and you would need quite a bit to flush properly so expensive if buying water the trick is to get your runoff ppm the same as the water you are using to flush with, in theory what 'we' trying to do in a flush is remove the medium of nutes so the plant begins to use the stored energy in its leaves. Me I just transport her to the bath tub, remove the shower head from the pipe, place pipe in pot and switch the shower on and flush that way but that's me. If your flushing because of salt build up or nute lockout then I would use dechlorinated tap water just for the amount you need to flush, hope this helps👍