Looks like overwatering plus lack of nutrients. Definitely get them out of that peat. The PH of that stuff is very acidic, sometimes 4. The acidity of peat is supposed to be good for seed starting because it inhibits fungal growth, but after that, your plants need to be in something more balanced, like 6 ish.
Sunshine growers mix is good, I have heard farfard 3b is also good. Or a bag of potting soil from home depot, just be careful about buying anything that includes time released ferts. It should drain quickly and you may need to add perlite or some aggregate to help with drainage if you go with a cheaper bagged soil.
Kelp is a good fertilizer, so is fish emulsion, but it stinks. Anything balanced and low-strength 5-5-5. If you want a soil that has nutrients included so you won't have to feed for a couple of months, I am having good results with fox farm soil. It is expensive but there is no guesswork, so it's a convenience thing for me.
You may need to remove the yellowed leaves with sharp scissors. They don't usually improve, but you can wait a while and see.