Auto GG4 first grow

I am unsure why #2 is starting to have issues, before it was doing just fine and #1 had issues. Their conditions have been almost identical so I am confused as to what is making them develop so differently. I believe water ph could be a culprit, as I know the area I live in adds alkaline compounds to assist in stopping corrosion, perhaps I will try lowering the ph of the water slightly. #2 yellowing is only occurring on new growth (2nd set of true leaves and 3rd at meristem), so I know it not a nitrogen issue. #2 is 3.75" tall. Slight reddening in apical meristem as well (3rd developing set of true leaves), perhaps an iron issue, which would be caused by ph. #1 is nice dark green, but slightly shorter at 3" height. Also, haven't got the watering dialed in 100% yet so this could be another factor. I reckon I'm more likely over watering than under watering. I've been giving them anywhere between 200-300ml each usually every 2-3 days.