Have you fed them anything? Jiffy mix has zero nutrients. Give a boost with seedling fertilizer. Not seeing top-growth doesn't mean there's no growth. Right now your seedlings are developing root systems and those are far more important to a seedlings outcome than too much topgrowth wth nothing to support it below the soil. Outdoors you're still experiencing cool nights and the sunlight hours aren't long. You shouldn't be seeing a lot of topgrowth, that would be unatural. Patience is a virtue, don't rush the development of your seedlings.

My seeds I start in a community pot and at two true leaf stage transplant into salvaged sixpacs. Up until they are ready to transplant into four inch pots they are bottom watered as needed. In the winter they are exposed to a high light source this time of year no artificial light is needed. If you feel yours are sub par while on artificial light I would suggest you are not using a strong enough light. I use 400 watt HID light source and regulate distance to plants with the help of a light meter. Al

Would wet roots account for the stunting and yellowing lower leaves? Yes. ;) Plus compost really retains water as well so the perfect breeding grounds for the fungus growth was created. I honestly don't think the problem is from using compost in your mix per se. I know many object to its use but many of us use fresh homemade compost in our seedling mix with no problems. But using the "old store-bought compost" could very well be the source of the problem. It was likely contaminated from just sitting around and all the beneficial microbes were dead or inactive due to age. Next time, just mix it into the garden rather than the seedling mix. Good luck with your plants. Dave

I compared your pics to these others on this website from Cornell University; it has an extensive list of possible causes. I wasn't able to locate anything that looks exactly like your problem. Is the distoloration on the bottom of the leaf also? You could try wetting one end of a Q-tip and supporting the underside of the leaf with a finger or two, gently rub the spot to see if it transfers or comes off. My thoughts are either a mineral in the soil, or fungus; most likely fungus. If the condition - coloration, saturation, location -changes or migrates to the stems, I'd pay a visit to your local AG extension office; they are your best bet. You might try placing a small fan in the environment to improve air circulation. One last thought, is your soil aerated well? If your potting mix is too dense next time be sure to mix in about 20-30% of Vermiculite to ensure good soil drainage and aeration. This will also produce better conditions for root growth. You may want to treat using a multi purpose fungicide that contains neem oil, but not a sulfer powder. Blessings! Here is a link that might be useful: Tomato Leaf Symptoms