Help With Leaves Dying!

Hi,im having trouble with the leaves dying on my plant,im growing it in Hydroton clay pebbles in a FloGrow with 3 x 15 minute floods a day,im using Coco A+B and Canna boost,my PH levels are 5.5,im using a 600w HPS with a CoolTube in a 120x120x200 tent,Temps are around 25C,for the last 2 days i have drained the tank and just giving them plain PH'd water.
I have attached some pics,any help would be appreciated.
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