The usual cause is the soil is so full of water there is no air at root level and the roots are unable to function. If you are sure you are not watering when it is not needed, then your soil is just not draining well enough. Al

I have not been over watering...planted them up in 3 inch peat pots. I bottom water them until the peat pot is wet and then don't water them again until the soil and pot is bone dry and some of the seedlings start to wilt just a little. I know you don't think you are over watering but you are or the leaves wouldn't be yellowing. And the peat pots are contributing to the problem which is why many of us won't use them - ever. The peat pots wick water up too quickly and then wick it away from the soil and the plant roots thus forcing you to water more frequently. When you water until the pot is wet it is too much water for the roots and the leaves yellow. Then the pot dries quickly and begins to pull the water out of the soil drying it out and making you think you need to water again. If you cannot get rid of the peat pots and move the plants into plastic cups which is the recommended solution, then you will have to water them from the top using a small watering can so that only the soil itself is watered until it is only slightly moist. Make sure the pot gets little of it. The check the deeper soil moisture level, not the pot condition, before watering again. Going from real wet to bone dry should be avoided at all costs. You want a consistent LOW level of moisture in the soil. Hope this helps. Dave PS: lots of discussions about the many problems with peat pots here you might want to read for future reference. And do be sure to strip it all off the plants before transplanting them.

The "primary" nutrients all plants need to varying degrees are nitrogen, phosphorous, and potassium. Without those nutrients supplied in some form anything else makes little to no difference. Neither peat nor coir supply any of those nutrients. Most brands of fish emulsion supplies all 3 in small amounts. That is why plants normally get transplanted to a growing medium of some sort that contains those nutrients. " though the plugs are bigger ones and lasted for a good 6 weeks last year. Seedlings are just over 4 weeks old at this point." Yeah I know the size of the plug flats you are using. We use many of them in the nursery but for germination and growth only to the first true leaf stage. Then they get transplanted so that feeder root development can begin and you begin to supply them with a diluted form of nutrients. Dave

"Balanced" usually means that it contains all the major nutrients N-P-K as well as some of the micro nutrients. There are hundreds of them but most common one is Miracle Grow Plant fertilizer mixed to 1/4 to 1/2 strength. Use something that is liquid or water soluble. For future reference all seedlings once they develop their true leaves need some supplements. A further problem is you are using those dang peat pots with all their problems. The main one being their appearance encourages over-watering. So also cut back on the water and get them out of those pots ASAP. As is often discussed here, please be sure to strip off all the peat pot before planting. Dave Jack types faster. ;-)