Can any one tell me where I'm going wrong here?

These were bought as Afghan Kush Ryder seeds. They have all been planted and growing exact same every thing from light to water grow medium ect. Will be 1month exactly on 13th. Not sure what is up with the small one being so small and they look wilted and changing yellow like they are dying as of past day or 2. Water from tap with filter and ph at about 6 with canna coco grow medium mixed with clay pebbles for extra drainage in 2 gal smart pots. Pics of nutes used at the moment attached and adding 1-2 dosage to water. Running 20/4 light cycle.

I had a few of those hanging earlier, but in soil. Try to go easy on the nutes and measure your runoff waters PH. In my case I just flushed it properly for two waterings. Just started adding 1/4th strenght nutes and plants are looking great again I would advice you to try it out cause those tips looks like nute burn, but the plant hanging could easily be salt buildup and acidic medium in my opinion. Either way, both problems are solved with a real flush or two. Remember, 3-4 times your bucket size in water. Flush it through and make sure you flush everywhere, not just in one spot. Good luck

Dang it sorry posting from phone meant it is 1/2 strength nutes. Ie: I use 2gal watering can it says 15ml per US gallon so I put about 15ml for whole 2gallon can.

Your in Coco?

Give coco a quick wash for the struggling plant. Looks like a salt build up/lock out condition starting as that plant is using less then the others.
ADD a Cal/Mag to COCO just like for RO in hydro!!!! Coco seems to need that! (for all the plants)
That should help her out.

Keep in mind that Coco retains salts more then soil. So a little flush a cpl of times in the run will help that.

Awesome so looks like trying a good flush or 2 is what needs to happen. This is first crow in coco was looking to step into soiless and hydroponics store assotiate hooked me up with current set up as he said was so mucheasier than dwc system that I was going to go with.