When I said burning, I was talking about the browning on the leaves. Nutrients are good but too much will cause what's know as nutrient burn.
As far as your schedule for light and dark time, it depends what stage the plants are in. Judgeing from the pictures they're in the vegetative state and need 18 hours light, 6 hours dark. Once you get the plants basically to the size you want, you change your lighting to 12/12 to force the plants to flower. Increasing the dark time replicates the short days of fall, causing the plant to flower aka produce bud.