Keep them on 18/6 I'd say man this will, as said above give your bulbs time to cool down a little and any residual stagnant air would also have the chance to be blown out and away from your room, I'd suggest leaving a vent open for an hour or so after the lights turn off to allow this to happen. As for nutrients it all depends on your medium, the water you are using to feed is it hard or soft and also your budget I'll be using Canna Terra Vega, Flora, Boost, PK 13/14, Cannazym, Rhizotonic, Canna Start (Which I'm not using), and Canna Flush. I'm only using these because I'm using the Canna Terra Pro + soil, 100% organic materials. soft water has very few minerals and a low PPM within it whereas hard water has more minerals and a higher PPM, the PPM is mainly for Hydro growers but can be utilised and monitored with a soil grow. the Minerals in the water tend to be things like: Lime, chalk, small particles of dust etc. and mostly Calcium and magnesium. you can make hard water softer by boiling it but if you go with soft water I'd suggest purchasing some Cal Mag or a similar product.