You shouldnt need to flush unless there is a ph problem or salt buildup.
You can start giving them water with just some molasses and some P-K in it for the last week or so but thats as close to plain water as id go.
that nug looks very happy but some of the leaves in the background look a little deficient so id just keep on with the nutes youre feeding now or step them up a bit for the last month then the last few days switch to molasses and just some p-k.
You can even start feeding once every week or 2 with molasses now and itll help. Especially if youve got some enzymes you water with.