for the last 7 -10 days of harvest use just water, no nutrients, and flush with 3 times the amount of the container size every month or when you switch cycles this prevents salt buildup in the medium
Another good flushing method is to check the trichromes first, and if they are changing colour, start your fresh water flush then. It is usually about that time that you really want any chemical residue to be flushed. Hope this helps.