The light off thing is kinda a myth. But plain water, with even a little molasses dissolved in it if you like, is perfect for the last couple days before harvest.
BTW there are a lot more hydro-oriented peeps on here than me lol. My hydro style is: open bottle of Canna Aqua. Dump in res. Walk away. Wait for plants to start looking crappy. lol! Kidding, but I don't really get fancy with my dro stylee.