Plant yellowing and deforming

[Frostage]

Hi all,

I'm now 2 and a half weeks into my first grow, and one of my plants is growing perfect. It looks nice and healthy and is growing quick:

My problem is that the other plant is showing some strange signs. The leaves all have a yellowish tinge, and they are beginning to crinkle up and deform. There is also some reddish tinge to the stems, and growth has slowed right down:

Please see my grow journal for more information on my grow, but here is the basics:

Medium: Coco coir & perlite
Environment: grow tent with 250W HPS, extraction system and cool air inlet. Temps at ~23-28C during lights on, 18-23 during lights off. Humidity fairly low around 40-50%.
Strain: Dynafem big kush (feminised, photoperiod)
Nutrients: Started with pure pH'd water for first week, then fed nutrient mix (General Hydroponics), noticed slight nute burn on the first leaves so went back to pure water for another week, then started a much weaker nute mix a few days ago.

Any advice would be much appreciated guys!

 

[Woogie]

Hi , I'm also in coco and the first time I switched from soil I had the same issues , thought it was ph first checked the runoff, everything was fine then I checked for bugs , no bugs, so I did a bit of digging and found the coco needs to be supplemented with calmag unless using coco specific nutes, once I started to add calmag to the mix the plants bounced back after a couple of days,
What I tend to do now is precharge the coco by running low ppm with calmag through the medium till the runoff is 5.8 then plant and feed ,sorted out my initial problems with young plants in coco.

Remember coco has no nutrients in it , with a week on just water the leaves will start to yellow, it's eating it's self
Just my 2cents
Hopefully some of the more experienced growers will chime in