Sugar breath

6 hours wont hurt anything even 24 wont hurt as long as it dont happen often. I am more concerned with the amount of plants you have in your room. Growing from seed takes at least 4 weeks of growing before they can flower if their photoperiod plants. Unless your room is huge I would be switching them to flower now or that room is going to be jam packed with plant blocking light from getting down and air from circulating throughout the plants. Looking good so far. Luck and Cheers

A few things to consider, I was under the impression 6.0 ph is a little low for soil, I have seen spotting due to low ph many a time. Also do you know what ec/ppm you are on? The amount of feed per L sounds very excessive for such small plants and you can see burnt tips on a few of the leaves in the pics. I woood consider dialling back the ec a touch. I know some are ok with a 6.0ph for soil but I would be in the 6.3-6.7 range