Seedlings yellow and droopy

Two seedlings have become colored light yellow or light green and the leaves are drooping. These are 2.5 weeks old. One week-old seedling (now as large as the other two), however, is doing fine in the same grow space. The only difference is that it was planted in different soil (indoor/outdoor organic).

Details:
Dimensions: (3 x 2 x 1.5) ft, mylar-covered walls
Lights: 2x 85w CFLs (total 10,000 lumens, or 3333 lm/ft²)
Light cycle: 18/6
Temp: 60-70°F
Vents: Passive intake, 20CFM exhaust fan (cycles the space ~2 times/min)
Watering: ½ cup distilled water every 2 or 3 days
Soil: Regular potting soil

No additional nutrients have been added yet.

Attached are two images.
Images 1&2: A yellowish 2.5-week old seedling.
Image 3: A healthy 1-week old seedling, for comparison.
-Note: photos were taken just after watering.

What do you make of this? Could the sick seedlings be nutrient deficient while the one in the organic soil is still getting nutrients from the soil?

I don't know what the soil is for the first two....but that looks very like abnormal ph and/or fert problems (like maybe too much ferts for seedlings problems) Also...are you overwatering? That will make them sag as well.....and mess with your ph at the same time.. If the third pic is the one in organic soil.....it STILL seems to have a few issues......but not as much as the other two. Be SURE you are not overwatering and have good drainage. If you are sure that's not the issue I'd try to transplant to better soil......sooner rather than later.

How much and how often should I be watering? The first soil is simply called "Potting Soil" and was pretty inexpensive. Also, I haven't used any fertilizer yet, so do you mean a soil fertilizer problem?

Many soils come with fertilizers already added.....that's what I mean by fertilizer problems. I grow hydro....not in soil....but do know that many types of soil on the market are too "hot" for seedlings.....ie...there are too ferts already in the soil which can overnute or burn young plants. Unfortunately I do NOT know which soils are the good ones vs the bad ones.....so can't help you with that part.

I hate it when people ask me how to water a plant.....I'm terrible at that kind of thing so will have to leave that for someone else. But I know the roots should not be constantly wet or they will not be receiving enough oxygen and that drainage is probably your biggest key to this.

Revised: The Complete Guide To Sick Plants,pH and Pest Troubles - PlanetGanja.com

This is a great link for plant problems and solutions. Hope it helps.

Good Luck. ;)

for seedlings, you should give em a nice amount of water evry day or 2, just enough to keep the soil at an almost constant moisture, since thats pretty much all the seedling needs until its root system can be made large enough to begin a proper dry-wet cycle

just make sure it moist, not wet, not soaked.

That guide weedhound gave the link to is excellent