Help with seedling Pls

Hi all, I currently grow a Tangerine Dream Auto by Barney's in the gemini plant it system. The pot size is 18lt with clay pellets and the tank below is 8 lt of water with the feeding solution. The day time period is 18 hours and watering schedule for the day time 15 - 20 mins approximately every 2 - 3 hours, night time there is no feeding. The grow room is about 0,50 square meters, and for light I use the 144 Mars Hydro Reflector Leds. Maximum temprerature 25 Celcius and humidity ranges from 55 % - 65%. I feed with Advanced Nutries and for the first week of vegetation I fed it with the 25% of the minimum recommended strength (0,25 ml per lt). By the end of first week I noticed some spots, which I think was caused due to low nutrient solution. The Ph is 6 and I try to keep it up to this level, meaning there are no great ph fluctuations. Pls guys every opinion will be welcomed.

It looks fine. Whats the ppm or ec of nutrient solution? Whats the temperature of your water? What kind of water, tap? RO?

I think they look good maybe just lower nute strength a bit

Only 8ltr of res for an 18ltr pot sound odd. You can allow some fluctuation in ph, start 5.8ish and allow it to rise to 6.2 ish, its better than constantly adding ph down which is phosphoric acid.
Also clay pebbles can dry out quickly, not feeding at all at night could cause you problems when the plant is bigger.

First of all I would like to thank all of you guys for the advice. Ph checked and corrected to 5.9. I use tap water, in the place where I live the tap water is great. I follow the Adv. Nutrients program and used for the first week the 25% of the minimum recommended dosage for seedlings(0,25 ml / lt) and for this week the 50 % of the minimum recommended dosage (0,5 ml / lt). Usually when the solution is that low I have some Ph fluctuations. Checked and corrected some things yesterday and this morning tha I checked she seemed happy. So here are some pics