First thing you need to do is test the pH and TDS of the water you are watering them with, then test the runoff.
They look ready to start feeding but can't. If the TDS is over 700 on a .5 meter the soil is too strong and needs to be flushed. The pH should be off too if that is that case because the plant is too young and not healthy enough to adjust the pH on it's own.
Usually watering a plant with around 7 pH water will cause the runoff to be around 6.5 or so which is where you want to be at. Carbon filtering some tap water with a Brita filter usually causes the water to come out around 7 pH.
I'm not sure if the soil you are using is organic or not... But usually potting soil is not for seedlings. It might just be that the soil needs to be flushed really good with some clean water with no chlorine in it.
I would definitely start but testing the source water and the runoff pH and TDS values though. If you can't do that, flush the soil really good with about a half a gallon of filtered tap water per cup. I would also look into some sort of b-vitamin, perferably superthrive because it's the least expensive and you don't have to buy a huge bottle of it.
Hit us back though, I want to know what's going on
Oh ya- and next time you start from seed... Use coco coir or some sort of starter plug like a Rapid Rooter. You can't over-water coco coir or rapid rooters. The only thing you do different is adjust the pH down to 6 when you use either of them, although I haven't really had a lot of trouble using water that isn't pH adjusted until they need to start feeding. They really work a million times better than soil for starting seeds or clones.