I planted some germinated White Widow seeds about 2 1/2 weeks ago and they popped up after a couple days and seemed happy. So I put them in my grow tent under my LED grow light set to veg. They grew to about 2 inches and stopped. Now they don’t look happy at all. I followed the instructions on the site - put them into Miracle Grow seedling mix in 4 inch pots and have kept them moist. Should I give them any fertilizers at all? What am I missing? This is just my second grow, so am still learning.
No fertilizer. Never until they establish a strong root zone.
We really cannot help you until you provide us with more info, like temps, and humidity levels for a start.
Oh yeah; A picture is worth a 1000 words 
Keeping them moist?
This seems to be one of the biggest misunderstandings for new growers, and they often keep the soil too wet and for too long and end up causing more harm than good, cannabis roots are not that unlike cactus roots and can drown or get water logged easily if the soil is too wet or not well aerated enough. It is best to drench the soil once in a long while and let the soil nearly dry out completely in-between. Judge this by the weight of the soil totally wet and then when it gets really light and feels nearly totally dry, only then re-water.
Temps are around 75 f and humidity is about 35% right now. And this is almost 3 weeks now. I have been watering only when starting to dry. The pots drain easily.
Looks like a severe pH issue, what is the pH in the root zone?
What is the best way to determine that? Test the runoff water?
Yes, the run-off, or get a good pH soil probe to get an idea of what is going on at the root-zone.
I think I have found the problem. My tap water,which is filtered well water, has always been a consistent 6.5, which has worked pretty well. I recharged my charcoal filter recently and didn’t think to test the water, I just did that and it is over 8. Looks like I need to use ph down before watering.
Can you recommend a good ph soil probe? I bought one and it sucks.
Unless you want to pay an arm and a leg, apparently the average more affordable ones are kind of hit or miss, as to the quality and reliability.
I got mine on amazon or e-bay, I think for less than $5, I got lucky. I think the type I got goes more for $10-$15, now and they still seem hit or miss on the reviews as to your chance of getting a good one that will work for a long time for you. Mine has been working reliably for about 3 years and is nearly as accurate as my digital pH pens.
This one looks identical to mine and seems to be maintaining a fairly high star rating, so maybe that says something about the quality: http://amzn.com/B00F52E5PY
And then there is this other one that doesn’t have as heavy duty of probes, but none of these cheap probes are very durable, so be careful with either one and don’t expect to be able to jam it into hard compacted earth and not damage or break it. This newer one is getting mixed reviews, but it is made by a well known company, and it has the added bonus of being able to read soil “fertility” which is its way of measuring the EC/PPM, and this extra feature would be very beneficial as well. This meter would be a nice extra to have around and would likely do great for cannabis in its light and airy soils.
http://amzn.com/B000RUI9QK