I started some seeds indoors. They are growing under t5 lights. They started great, but now a couple of them, their leaves have turned yellow and started wilting. Am I doing something wrong?

First things to check out are whether you've watered too much (or too little but overwatering is more common), or given too much fertilizer. If it's none of the above then damping off could be a possibility although that can also be encouraged by overwatering.

Is there any pattern, i.e. are the sick ones close to each other or far apart?

I water from letting them soak up the water rather than from above. Havnt fertilized yet. Been waiting to see the peat moss start to turn light color before watering.

Can you post a picture of the affected seedlings? Or, failing that, how tall are they (how long since germination), and when they wilt is it the whole seedling from the base? The pattern question (adjacent or widely spaced) would give a clue to the cause also.

I guess around a couple weeks since sprouting. No real pattern to affected plants.

Is it plain peat moss that they're growing in, or a peat based seed starting mix?

Plain

They need to be fertilized don't they

Yes, but they're so young I'm not sure that's the problem, they should still be living on the stored food in the seed. Also it wouldn't cause wilting, although I'm not sure that they are wilting as opposed to the outer leaves declining (based on the picture it looks like there is an emerging new leaf that looks normal).

Usually peat moss is just part of the mix for seed starting, along with other components such as perlite or vermiculite to add more structure and maybe improve drainage/aeration depending on the texture of the peat.

Peat moss also has problems being wetted (which is why commercial peat-based media often contains a "wetting agent" ). It can happen that the top of the media dries out after wetting but at root depth it is still soggy, so you could check that out.

If it's Canadian sphagnum peat moss it will be quite acidic, normally dolomitic limestone would be added but I'm not at all sure that would be a problem for plants this young and in containers).

Peat moss does have little fertility of its own so you will ultimately have to start fertilizing but my guess is the yellowing has more to do with water or aeration. Can you check that pot with the unhappy seedling to see if it is dry or wet lower down?

I have a water meter I could put down in it and see

It says it's damp but not wet

Would they survive a transplant to better soil or too young

They should survive a transplant, especially if you're careful with the roots. Maybe just do a few in case those are the only two with a problem, and keep an eye on the others. If no others start to yellow then it may just be an individual seedling thing. How many hours a day are the lights on?

15 which may be too much

I would not expect 15 hours of fluorescent light to be too much light. I had daylilies growing under 24 hours of incandescent light last winter with no problems.

Would an indoor potting mix be ok if I can't find any seed starting mix?

I agree with Maurice, 15 hours isn't too much. How close are the lights to the tops of the plants?

A good quality indoor potting/growing mix should be OK since the seedlings are already up and running. It will likely contain fertilizer so you won't need to fertilize for a few weeks (it may say on the label how long they expect the incorporated fertilizer to last).

About 2 inches from the tops

I have some seedlings under light 24/7. I also have germinated seeds on heat mats while under light 24/7.

I have also germinated seeds in the fall on a windowsill with no additional light or heat. They do take a little longer to germinate.

There are more variables that I will try this fall.

gargoyl52 said:About 2 inches from the tops

I assume it doesn't feel uncomfortably warm when you put your hand under the lights?