Hey Mortaion, so sorry to be late getting back to you on this. I agree with The Hemp Goddess that they are suffering at this point from micronutrient deficiency. When you flushed the soil, it must have stripped it better than we thought. With the damage that they suffered in the beginning, they had to use up their cotyledons sooner. So now it is time to feed them a bit.
This is where you must be extra carefull.
The nutrients that you show there look like the correct line of nutrients for the vegetative stage but don't go by the directions exactly. What you need to do is look at the directions and see if it gives specific directions for feeding very young plants. If it doesn't say anything about young plants, and it only says to give 20ml per 3L water, that is for adult plants. What you will need to do is get a container that you can keep the water in for watering/feeding them. Then fill it with the 3liters of water and only add 1/4 of the total to the water. That means for right now you will only add to the water 5ml of the nutrient solution.
You will mix this thoroughly and allow it to set for 1hr then check the PH of the solution. Then wait for 6hrs and check it again. If the PH does not change(and it is at the proper range of 6.5-6.9), it is ok for using to water/feed the plants. If the PH does change then you will need to wait another 6hrs and then check it again. Once the PH has reached a point where it isn't changing anymore then you can add in adjusters if it is outside the proper PH range(or use it if it is in the acceptable range).
If it isn't at the proper PH then you will need to adjust it to the proper PH by adding PH adjuster chemicals(acid buffer to lower PH and alkalinity buffer to raise PH). Then you will need to mix thoroughly and allow it to set again for 1hr and then test PH again. You may have to do this several times to get it right. Start by adding small amounts like 1-2ml of buffer chemicals at a time so that you don't over-shoot the target PH as it isn't good to be adding in both types of buffers to the same solution. After you have done this a few times, it will get much easier, and you will be able to skip steps as you learn how much of each chemical affects the solution PH.
Once you have the water/feed solution ready, you can add to the soil of each plant just enough water to wet the soil good. I wouldn't soak them to the point that they drain water unless the soil is drying out fairly quick. Keep your solution for future watering when the soil gets dry. With this type of watering/feeding, you will be feeding them every time you water, which is ok to do when you don't have soil with nutrients in it, and your solution is mild.
Use this level of nutrients in your water for about 7-10 days unless you see significant growth withing 5 days. If you do see significant growth then when you need to make up another batch of "nute" solution, you will increase the level of nutes to 10ml to 3 liters of water and continue to water/feed as before. After 7days at this level, you should see continued significant growth. If no problems occur to this point and the plants are looking healthy you can increase the nutrients to 15ml per 3liters at the next batch of water/feed solution you make up for them.
Each time you increase your nutrient level in your solution, give the plants 7-10days to react to the "nute" level before increasing again. This will allow you to monitor their growth and see if any problems occur. You will continue to increase the level of nute in the solution until you reach 20ml per 3liters or until you begin to see the tips of the leaves begin to burn again like they did at first. If you see this "tip burn" on the newer leaves, or the leaves begin to curl under as if forming the the shape of predator bird claws, that means you have reached the maximum amount of nutrients that the plants can handle, and you will have to use 5ml less in next batch of solution. If you have a fresh batch of solution and the plants show max "nute burn" or "leaf curl" then just dilute the solution with 1 liter of water rather than disposing of it.
I know this is a lot to take in
but it gets easier over time as you get familiar with the needs of growing MJ. As you said, growing MJ is not at all as simple as one would think. Many, many new growers discover this and many end in failure because they are unwilling to embrace the needs of growing MJ. It is far more involved that one would think, but not as difficult as one would fear.
