First, I'd cut back on the lights. They don't need 24/7 light exposure. 16 on and 8 off is plenty of light.

The tap water you've been using is in the normal range for drinking, so I doubt that's the problem. But using distilled water won't hurt anything, of course.

Second, can you check to see if the plants are root-bound. Peat pots can prevent the roots from extending into the surrounding soil. The roots can act as if they're nutritionally starved when that's the case -- they just cannot absorb much of anything, including water or food.

And last, you haven't put the plants outside without hardening them off, have you? You didn't say so, so you probably didn't, but I thought I'd ask, just in case.

The pH of the water doesn't pose any problems. Millions of transplants are grown annually with plain old tap water. ;)

The peat pots and the moisture control problems they cause are the likely source of your problem. The pots need to be stripped off before transplanting. Not only do they cause rootbound plants but they wick the water away from the core of the soil yet retain too much water in themselves leading to root rot.

I would suggest you UN-transplant them, strip off the peat pot, and re-transplant them back into their plastic containers. Give the plant 24-48 hours to readjust, then give them a mild 1/2 strength feeding. Reduce your watering by 1/2 - the yellow leaves is classic too much water symptom. Do not water until the soil down to the middle of your finger is dry. Too much water kills more transplants than all other things combined.

Dave

PS: Oh and I agree, cut back on the lights to 16-18 hours max.

We're having our own tomato seedling problems, but I'm curious about how you're doing the pH measurements. By definition, pure distilled water is exactly neutral pH, which is 7.0 so either your distilled water is contaminated or there's an error in your measurements.

I am experiencing something similar. I placed my plants outside for about 6 hours on two back to back days (because I realized where I had them for the prvious week was not getting any light). The next day they started turning yellowish and are now drooping, so I started watering about every other day. The peppers I have with them are experiencing somewhat similar behavior.

Is it bad to have tomato seedlings in direct sunlight (outside)? How often and how much should I be watering seedlings (example: 1/x cup of water every x days)? Is there any hope for these plants or should I just cut my losses and start over?

Digdirt, I started my tomatoes in peat pots.
If this isn't good for them, what do you suggest I use next year? I have a very small area to start them indoors, not really enought light. The area is small.
What kind of containers are best to start tomato seedlings in? I haven't seen any small plastic pots sold at the store, and want to make sure everything is sterile.
Perhaps I could save small plastic food containers, like yogurt cups, and poke holes in them?

If the nutrient solution within the root mass has a pH of 8.5 the tomato plant will experience significant nutrient deficiencies and eventually die.

Adjusting the pH of your tap water is a good idea. Often it is better than using distilled water.

Distilled water has no buffering capacity and therefore does not maintain a stable pH with even the slightest contamination which is why you will get wildly fluctuating readings when checking the pH of distilled water. In comparison, your alkaline tap water, high in bicarbonates, will resist changes to pH.

At a pH of 8.5, most of the nutrients in your soil will precipitate out of solution and become unavailable to the plant. The ideal nutrient solution pH for tomatoes is 5.5 to 6.5.
The pH scale is logarithmic, therefore we can see that the ideal nutrient solution is 20 to 30 times more acidic than your tap water.

You can use "pH Down" or any other acid, even vinegar, to reduce pH to the proper range (not pH down for swimming pools).

My native soil has a pH of 8.4. Vegetables will not naturally grow here. I use battery acid, sulfuric acid drain cleaner, ammonium sulfate, aluminum sulfate and elemental sulfur to grow a very productive garden.

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nice pics

If your distillerd measures 5.5 something is wrong with your tester. Distillerd water is neutral at 7.0.

Distilled water pH not 5.5

Here is a link that might be useful: pH of Distilled Water

Forgot the ? mark. It is explained at the source linked that pH of distilled water could be about 5.5, because of the combination with carbon dioxide from the atmosphere, forming carbonic acid.

I just wanted to add that Dave's concerns about your peat pots are well founded. I recomend removing them also.

Digdirt, I started my tomatoes in peat pots.
If this isn't good for them, what do you suggest I use next year? I have a very small area to start them indoors, not really enought light. The area is small. What kind of containers are best to start tomato seedlings in? I haven't seen any small plastic pots sold at the store...Perhaps I could save small plastic food containers, like yogurt cups, and poke holes in them?

The main point I was making above got lost in all the water pH discussion. ;)

The primary problem is too MUCH water - thus the "cut your watering by half" comment. Yellowing leaves and limp lanky plants are most often a result of too much water and the resultant root rot. And watering them more only makes the problem worse.

Peat pots further compound the watering problems as they wick up the water from the core of the soil into the pot and keep the outside edges where all the fine roots are developing too wet and kill them.

There are umpteen things made of plastic one can use for growing plants in if you don't want to buy regular seed starting flats and/or cell packs which are not only cheaper than peat pots but readily available both in stores and online and re-useable. Around here plastic seed starting trays are sold at Lowe's, Home Depot, Ace Hardware, WalMart, Dollar General, etc. etc. and they can easily be ordered online as well.

But many use plastic butter dishes, yogurt cups, cottage cheese containers, plastic or foam drink cups, etc. Just use your imagination and be sure to poke drain holes in them. ;)

and want to make sure everything is sterile.

"Sterile" is not an issue. Millions of plants are started in greenhouses all over the country each year and believe me - greenhouses are NOT sterile places. Clean is all that is needed. Seed starting mixes are as sterile as they need to be. The fungi that cause damp-off is in the air around us all the time but it is not a problem unless you over-water which 95% of us do.

Damp-off is caused by over-watering, too warm growing conditions, and inadequate air circulation around plants. All of these points are discussed in great detail in the many FAQs and discussions over on the Growing from Seed forum here so be sure to check it out.

Dave

PS: An easy way to prove to yourself that you are over-watering is to split your plants into 2 groups. Water half your plants as you would normally and the other half of your plants half as often and at half the amount. You'll see the differences within 1 week and you'll never over-water again.