HELP!!! 1st hydro grow, yellowing leaf tips

Howdy all... Any and all advice would be appreciated. I know this could be the start of almost anything so let me give you some background info: 1st hydro grow in a superlocker, 250w HPS, Temps 75-81, humidity 45%, pH 5.8/5.9, nutes: TechnaFlora's recipe for success at 50% strength, 2nd week of veg- 5th week from seed... I've had some minor issues with pH and nute burn but those have been successfully corrected. My ladies look generally happy, no droop, good growth. I'm using a combo DWC and top feed system, although with the plants being young I've yet to use the top feed pump, it's been only the DWC so far. I recently noticed my leaf tips have begun to yellow. What could be the cause??? What could be the solutuion???

I appreciate any help and advice, good karma
 
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