This is only my first year messing with any of this stuff, so take everything I say with a grain of salt. That said, I just put my tomatoes through a very similar set up, and had a similar problem.

I used an 8 bulb t5ho light, and kept it about an inch off the top of my plants, on an 18/6 schedule, so I too think it's not likely to be a lighting problem, for what that's worth. I also had a box fan, on low, on the plants for 45 minutes, every other hours, and they did fine.

I started my grow with age old fish/kelp fert, and fed every 3rd watering at a diluted rate. The plants thrived, but then I ran out, and couldn't get more quickly, so I picked up age old grow instead, on a recommendation. When used at similar levels of dilution to the fish ferts, it killed 18 pepper seedlings, and all 12 of my tomatoes were looking a bit saggy, and showed yellow new growth.

I thought to myself (remember, I'm still new) that since the new growth was yellow, the tomatoes needed more nitrogen, but I didn't want to risk all of my tomato plants, so instead, I just gave the most yellowed one a strong does of the age old grow, to see if that fixed the problem. Instead, That tomato plant went completely limp, and lost all of it's leaves. Since then, I've stopped using the age old grow, and with time, and plenty of water running through the pots, the tomatoes have turned green and healthy again. The peppers, on the other hand, all had to be replaced. On the recommendations of those over on the container forum, I'm now trying Osmocote for half of the replacement peppers, and Earth Juice Grow for the other half (my experiment to see if I can do this successfully using organics).

Long story short, when my tomatoes showed similar symptoms, in a nearly identical situation, it was because I was using too much of the exact same fertilizer you are using. Discontinuing it's use, and fertilizing more lightly, with a different, weaker fertilizer (3-1-1, vs 6-1-1) in the future, has saved many of the effected tomato plants.

I can run outside (the tomatoes have since been transplanted into soil) tommorow, and take pictures of the residual yellowing, and the nearly dead plant, if you are interested in comparing to how yours look. Mine didn't look like traditional fertilizer burn, but that's what it ended up being.

Thank you both for the replies! Sorry for such a long post the first time. I will try to be more brief in the future. I never really did figure out what killed the plants. I decided it was most likely a disease of some sort. Not sure how it would have made its way to my basement though, I used all fresh starting mix and clean pots etc. Some of the tomatoes, about 6 of them, survived the ordeal. I still have them, some are doing ok and a couple refuse to grow at all, just sad stunted little plants. As far as the questions above: I am using ProMix BX as a starting medium. The tomatoes have been grown purely in my basement (consistent low 60s temp, dry air). I do not know the pH as I have not been testing that but probably should be. Do you recommend a particular test or device for that? Sean, strange that you had trouble with the Age Old Grow?? I had not thought about that. I am conservative with fertilizer but maybe should try a different kind just to be sure.

I have included pics from the time of my first post, before the plants went into the garbage. Let me know what you think! Thanks again.

"I started 2 flats of tomato seedlings in mid-February"

That's absolutely terrible. I plant tomato seeds around April 1 and the plants are always ready for planting outdoors within 6 weeks. See pics at http://forums.gardenweb.com/discussions/3018913/failed-led-strip-lighting-experiment?n=1 .

"It is softened water but I switched to potassium chloride as the softening agent after last year's issues."

Isn't softened water strictly for cleaning laundry? Otherwise, why use softened water? If I were you, I would use bottled distilled water on one or two plants to see whether it made a difference.

Also, you are wasting money and unnecessarily exposing yourself to variables outside of your control by using pre-mixed potting soil. I use a homemade mixture of 2/3 peat moss and 1/3 Riceland Parboiled Rice Hulls. Right now, Menards is selling peat moss for $5.98/2.2 cubic feet. Rice hulls are even cheaper, if you can find a source, which is admittedly difficult. Compare that with whatever Promix BX cost you. And, carefully read about my fertilization practice at http://forums.gardenweb.com/discussions/2005610/how-to-easily-supplement-fluorescent-with-incandescent-light .

Well- I started them in February as something of a trial run. I had trouble last year with seedlings and had made some changes I wanted to test out .I figured if they did well I could continue to pot them up and leave them indoors until it was safe to put them outside in the greenhouse (early to mid June around here, if we are lucky). And if they didn't do well- I still had plenty of time to start new plants, which I have done. I know some parts of the country are very unfamiliar with water softening. Not just for laundry, in areas with hard water like ours the minerals will destroy your plumbing fixtures. So most people have all their domestic water inside the house run through a softener. I agree testing some plants with different water is a good idea, but I have heard distilled water is not good for plants?

I will look at your fert practice, thanks. Oh and the Promix BX cost me about $15 for a giant bag. It is a well regarded seed starting mix and the plants did amazingly well in it- for the first 4-6 weeks. So unlikely it is the problem. I have very little free time so a pre mix is really helpful at this point. Thanks for the comments!

"So most people have all their domestic water inside the house run through a softener" - Can you tap into the water before it gets softened? I suspect that your water goes to a water meter before it goes to equipment that softens your water. Maybe, there is a faucet in between? Do you water your lawn with softened water?

"I have heard distilled water is not good for plants?" - That's ridiculous. The only difference between distilled water and rain water is that rain water is more oxygenated. I have used stale, unoxygenated tap water (from Lake Michigan) on plants without problem. You just have to be more careful to not overwater.

I live in a rural environment in the Rocky Mtns and my water comes from my well. The outside faucet water is not softened. It is rather inconvenient to have to constantly go outside, get water and then haul it through my house and down to the basement. But it's doable and I will get experiment with that more. And I don't have a lawn, they are water hogs and we are in a drought! Thanks for the distilled water info, I will keep that in mind.

Think of your plants as canaries in a coal mine. It's important to find out what's killing them because your health might be at risk.

But, I have been phobic about drinking well water ever since I saw the movie, The Curse. See http://en.wikipedia.org/wiki/The_Curse_(1987_film .

I don't grow mine from seed. I just buy them from the Garden Center. BUT, the canary in the coal mine is a great HELLO! WAKE UP! You got a big problem with your well water. Check it out. Don't go stupid. We plant roses next to our vineyard rows. The roses are the canary in the coal mine. The rose will show damage prior to whatever it is attacks the vines. The vines are a living.
We are in the same drought as you, and we have a whole house water softener. We do have seriously hard water. It isn't from a well. It comes from the mountain lakes. We soften our water with potassium chloride, not salt.

I appreciate all the concern! In fact I did test my well water. It came out fine. I live essentially at the top of the watershed for a large portion of the western US. While not immune to problems I would bet my well water is many times cleaner/healthier than what the majority of the US gets in their municipal tap. Keep in mind aruzinsky that a modern water wall is a closed unit and very far underground, pulling water filtered through many layers of rock and sand. It is not a well like Jack and Jill went up the hill to fetch a pail of water! Suzi I also use the potassium chloride instead of salt now. Seems healthier all around. I will add that my poor neglected house plants all look fine and they have been watered solely with the softened water. But perhaps they are more resilient than a little seedling. I agree it is important to figure out what's killing them but mostly for my sanity! I don't have time to waste with entire flats dying for no known reason. The 6 bulb t-5 lamps do generate a fair amount of heat, I am wondering if that could have anything to do with it. I thought tomatoes could take a decent amount of heat though.

You should appreciate the fact that something is unusually wrong and you are not going to find anyone who can diagnose your problem. Potential causes of your plant's deaths include unusual occurrences such as a disgruntled employee dumping weed killer in your Promix. It is up to you to determine the cause of your plants' deaths by experimentally changing the variables.

Most novices don't begin with 324 watts of T5 fluorescent lighting. Typically, they begin with one or two $20 T8-T12 shop lights and gain some expertise and confidence before investing in a larger set-up. To save on your electric bill, you can perform your experiments on a smaller set-up.

I do appreciate the fact that something is unusually wrong! In fact I just finished discussing the problem with my county extension agent. She will be sending the pics I have to a plant disease specialist. I will get to the bottom of this!

I did not start with the big T5 setup. I also have a smaller, 2ft 2 bulb T5 setup I have used for several years. It has always worked great and I had lots of success. I got the bigger setup to have more space and more power to hopefully grow some veggies like lettuce all winter long. As this is Colorado and LOTS of people grow weed indoors, grow light setups are easy to come by!

Are you certain that the "mold" is mold and not dried salts? Watering only from the bottom could cause fertilizer salts to accumulate on the top surface of the soil which would damage the stems at the soil line. Check for stem damage at the soil line.

Well there is definitely green algae that forms after awhile, but the soil surface does get kind of crusty eventually as well. I have noticed the stems do look different right at or below the soil line, but figured that was because they were in the soil. The stems look a bit thinner and all brown as opposed to green like they are above the soil line. I would think that if the damage started at the soil line, the whole plant would be affected at the same time? It is definitely something to consider. Thanks for the help!

I just noticed that this guy has a problem similar to yours,

http://forums.gardenweb.com/discussions/3037110/help-tomatoe-seedling-looking-bad-pics?n=7

and both of you are using Promix products.