My wrinkly sickly plant

I have seven plants and one looks like it is shriveling up. Really wrinkled and just poor looking while other plants flourish. Really would like to know whats going on.

I have a 18 gallon dwc with six sites in an enclosed growroom properly ventelated
temp-78
humid-30-40
lighting-150 w hps one and a half feet from plant tops
water temp 68-70
co2enriched air regulated by control valves
70 lpm air pump so plent of oxygen
week two and a half
Only nutes are flora series at half reccomendation (gradually increasing)
H202 is used every other day and I change res once a week
ph I set at 5 and it rises to like 6.5 before I put it back down to 5 but I definitly stay on top of the ph!
ppm-678
I also have a window a/c unit to make sure things dont get too hot during the day.
and no guys, no root rot goin on.
PLEASE HELP BEFORE I MURDER THIS ONE