what do you think this is?

If it’s starting at the top it’s most likely an immobile nutrient deficiency, and most of the time it’s due to the ph being to high.

let me ask another question, are you using tap water?

any problems i have, the first thing i check is pH. cause thats easy to test. if the pH is cool i would back off on nutes a bit. But a vegging plant should be able to handle a healthy dose of nutrients. if it gets Calmag regularly then you might consider a flush

the good thing about growing in water is you can see the current pH youre running with and make corrections. with soil its kind of an after thought.

i just recently went through something similar,but it was all leaves, but it was my ph probe on my guardian went out of calibration. straitened out the ph and back to great healthy plants.so check ph first. gl   

you can use a soil probe to test the PH, you can also ph the water your using then catch some of the drainage water and test it.  there is an equation to show what your soil ph is but if I told you it would be wrong.  

Everyone on above said exactly what I was thinking. Anything I spot an issue the first thing I look at is the ph. 

Exactly what and how are you feeding this plant? Not only is the new growth have yellow tips but also this early on, having purple stems. To me it's one of 2 things.

pH issue at the root zone or the yellow tips has me leaning more to a magnesium deficiency.

OR... do some testing for a high EC ! The dark leaf color with purple petioles is a good indicator of ...

Nutrient Burn – Symptoms

Early signs of nutrient burn may not be obvious to inexperienced growers. First, the leaves of your plants will turn deep green except for the tips, which will appear bright yellow. The tips of the leaves of your marijuana plants will also curl up. As nutrient burn continues to develop in weed plants, the yellowed, curling leaves will begin to die and your buds will begin to turn yellow or brown.

False Positives – Similar Problems

There are several issues that may look like symptoms of marijuana nutrient burn but are actually indicative of a different problem. For example, light stress can cause leaves to begin to turn yellow at the tips. However, this will occur only on specific parts of the plant, while nutrient burn happens everywhere in the plant. Potassium deficiency and pH fluctuations can also cause leaves to turn yellow or brown, but with this issue, you won’t see your weed plant leaves curling up.

soil P H 6.3~6.8 when i used soil ....