Help please seedlings are stunted!

Beginner
#1

Hello, I’m growing some Diesel Fem and they germinated 3 weeks ago. They all sprouted to about 3 inches tall and its been 3 weeks and the only have 1 set of additional leaves only about 1/4 inch long. They seem to be stunted to what looks to be 1 week plants, and I’m not sure what is causing it. They are outside for the day in SoCal and brought in during the night so they don’t get exposed to low temps. Notice some purple on the stems.

Anyone know what could be causing it.

Thanks

#2

Keep them warm, 75 80 f. under led 100 watt inside, in two red cups inside each other, holes in bottom of the inside cup. small pebble under inside cup, potting soil. Damp…do not flood with water. No fertilizer…yet. T spoon Liquid seaweed 1/4 T spoon molasses in 1/2 gal RO water. Distilled water.
If the cup feels heavy…don’t water.