In this forum about 1/2 way down page is a thread called Zinc Tox/nute Burn...one of the posts in that thread (by Dutch lover) has a link to plant problems that is excellent. I tried to copy link but I don't think its going to work because I suck at that kind of thing.

International Cannagraphic Magazine Forums - The Complete guide to Sick Plants,pH, and Pest troubles!

Hope it helps.
Weedhound Reviewed by Weedhound on . Plant trouble, Need help. I have a Ebb& flow system with 3 plants in it, My plants have been on 12/12 light for about 3.5 weeks. Plant tray is flooded every 4 hours for 4 minutes, just enough time to fill the plant tray for 1 minute, while totally full. I am useing 4.25 OZ of Botanicare Power flower, 7.0 tsp of KoolBloom, and 56 ml of Hygrozyme for 7 gallons of RO water at about 5.5-6.0 PH. Only one plant is looking bad. This trouble is only in the middle part of the plant mainly. Little up top but not much. Any Rating: 5