Do you pH your water before giving it to them? my tap water after a 24 hour sit is still at ~8-9 pH ( I'm still using the General Hydro ph solution, no ph pen here. ) so I have to lower the pH for my plants to like the water.

I start flushing when I start to see brown pistils, ( this is because I am lazy and don't keep a proper log of what exact flower day they are on. ) but it all depends on the plant. If your plant has bulkened enough for your liking, then go ahead and start the flush. ( use pHed water, flush until you have barely any ppm in your runoff (100-400 is what I go by), then for the last 7-14 days just use pHed water. )

hope this helps.