Newbie to RIU. 2nd grow

Been reading for years u guys are all great. Got a 2x3 tent running a 4 beam 2ft t5 and a 600w fullspec LED. LSD, 2 Critical Kush, a holygrail kush and a crystal blue growing right now. 3 or 4 will be going outside when the weather gets better. Been experimenting with LST, topping etc.
 

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Quick question for anyone that can help. All plants are growing wondefully and green except this HGK. Its green and its growing but the leaves are curling and leathery looking. What could be the issue?
 
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