leaves yellowing at bottom - need advice

[Art99]

Hi this is my first post. I have 6 plants that are 3 weeks old and yesterday I noticed that the lower leaves on 2 of them are yellowing. In the middle area sort of a lima bean color and at the top dark green. My soil mix is about 1/2 regular potting soil and 1/3 vermiculite and the rest is perlite and peat. I don't have a real great PH meter but soil looks to be slightly below 7. I have not fertilized yet so overfert is not the prob. I started these under floros the first 2 weeks and then switched to a 430 watt HPS light with a Son Argo bulb. The plants are about 5 inches high and bushy with several sets of leaves o n it.

I figure it's either that it needs fert now or that my HPS light is too strong maybe. I can't see how a PH of 6.9 or so could be a source of a chronic problem but will try to lower it a bit. I was thinking about raising my lights a little and going down to 18/hrs day from 24 hrs/day to see if that may help. Not sure if I should fertilize a weakened plant.

If anyone can help with some advice it'd be greatly appreciated and thanks in advance..

[obejii]

picks would help but it sounds like nute burn

[Art99]

Sorry my cam broke last week and am trying to borrow one It can't be nute burn cuz I never used any fert on them and they were doing fine til 3 days ago. The only thing i've ever done is use plain distilled water on them. I think it's lack of fert probably. Got some Fox Farms fert yesterday and sprayed them with a mild solution and will do that for a couple days and see if that helps. Noticed the leaves are starting to curl too and from what i've read that's from nute deficiency.

[Art99]

Here is a pic. The lower leaves used to be dark green like the ones on the top. Don't know if you can see that the leaves at the very bottom are yellow and most are falling off the plant.