Purple stems and slow growth new EBB and flow

Hello Everyone,

New to EBB and Flow growing, using a DIY system with a 2'x4' flood table and 15 gallon res. My clones were taken from the plant 35 days ago and originally started in soil. After about 17 days I transplanted them into the EBB and Flow table and they have been there for 20 days.

Room Specs:

AN Sensi Grow A&B PH Perfect. Was at 900ppm and saw some slight burning so backed off now running at around 350ppm
PH stays 5.8-5.9
I run 1/3 cup 29% H2O2 per 15 gallons every 3-4 days
Water temp is now 68, but was not registering on the thermometer with a lowest reading of 64 until I added a heater 2 days ago
Room temp is between 68 and 75
4' 4 bulb T5 sitting 3" from tallest plant and maybe 6" from shortest running 24/0
6" square pots with just Hydroton

I only have an analog timer that mins out at 30 minutes which is set to flood 4 times a day every 6 hours 24/7

Tallest plant is about 10" tall and the main stems are purple on the plants. The leaves are all pointed up and I have roots coming out of the pots

My question is would the purple stems and slow growth be from the cold water up until two days ago? If so, should I just wait and give them room or am I not possibly watering enough and/or too much?

Thanks for any help

looks like a phosphorus deficiency. cold temps can do this. it will grow slower than potential, but itll be fine.

Will the heater I added to the res correct this eventually?

Phosphorus deficiency. I had this issue with one of mine, and oddly enough she's now the most robust. Just correct the problem and she'll be fine.