Yeah, man, your roots are shutting down and the plant is drowning, probably because of a pathogen attack in low dissolved oxygen (DO) water conditions.. The reason it is only happening with the smaller plants (so far) is probably because the bigger plants are more established and able to ward off attack better.
Follow Roseman's 8-step recovery. I would also recommend to STOP using the Liquid Karma (organic nute) until you get this under control, and adding 2 tablespoons of 3% hydrogen peroxide per gallon to your nutrient mix until the plants COMPLETELY recover. The H2O2 will help to stave off the nasties and increase the DO in the water.
Don't add the H2O2 full strength directly to the water with roots in it. If you can, take the plants out of the res and then add the H2O2, or drain out half of the res into a bucket, add the H2O2 into the bucket to dilute it, then add the water back to the res slowly. And you'll need to add H2O2 to the top-off water at the same rate - 2 tablespoons 3% per gallon.
The plants should perk up pretty rapidly, but I would keep up the H2O2 regimen for a couple of weeks at least.
If the roots are in the water, then turn off the water pump; this should decrease the temp in the water. The lower the water temp, the more DO in the water, and the harder it is for the nasties to survive.
