Help! 2 nirvana bubbleicious dwc

I have 2 Nirvana Bubbleicious seedlings just under a 100w cfl 6500k colour spectrum each, in 3 gal dwc buckets, they are about 3 weeks old from planting the germed seed in the coco coir (in rapid rooters). They both just seemed to stop growing in height! I know they should be much bigger at this stage of life, not to mention they are starting to turn purple in the leaves..., could this be a temperature problem?? All help needed please

Hard to help when you don't specify the temperature range you are running in or include a picture. Purple can be associated with a lack of potassium and phosphorous. Cold temps can lock these nutrients out. Purple can also be in the genetics.