Time to cull and let the new ones take over......
I didn't wanna do it but I have to admit defeat with these 3. I'm thinking I over-fed them plus used unfiltered water and that led to this sorry state of affairs. 3 newly popped seeds sprouted...
I've also got a couple of auto's arriving tomorrow.....need some weed and they're ~60 day finishers.
Using filtered water from now on with a basic compost that won't have much in the way of N in it. Hopefully it'll be easier to control the nute levels this way. Will use tiny amounts of Mega Crop when the time comes to feed and increase it slowly, rather than the stated dose on the feed chart from the beginning.
Hopefully I've made enough fuck-ups for the time being.........
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If they are not already culled, then those plants are salvagable.
You've got a few things going on there that have all been touched on by others in this thread, so you need to look at what has gone wrong as a whole.
Your seedlings are green and healthy - as were your original plants before you fed them - which tells me there is plenty of nutrient in the soil you are using.
Your pots were way too big - as was pointed out earlier - and as a consequence, your plants are suffering moisture stress because every time you water, you are soaking a large area that takes time to dry out naturally because the roots haven't filled out the pots and are not wicking away moisture. Temps are not especially high which won't help either. All this while you've been trying to flush away the damage of overfertilising . . .
. . . because one thing about having large pots is that there is a lot of nutrient in all that soil-peat mix that the plants can use once the roots find their way through it - so there is really never any need to feed a plant until it has started to exhaust its base supply of nutrient in the medium (soil).
I'm not a big fan of pot bags, either, because every time you pick them up to move the plant, there is the potential to damage the root structure. Air pots would be better, IMO - but nothing wrong with normal pots with good drainage.
Generally there is nothing wrong with using tap water. It is also hard to give your plants too much calcium - in fact, plants generally will show signs of pH imbalance (high pH) with excess calcium carbonate (limestone) before showing signs of calcium toxicity.
On December 1 (post 167), your plants were looking fine. On December 10 I see leaf twisting and clawing from excess N - always the fastest-acting element - and on December 13 I see P toxicity leading to leaf margin burn and calcium, magnesium and iron lockout.
Phosphorous is the devil, because when you overdo it, it leads to nutrient lockouts that look like deficiencies, and so the natural tendency is to continue overfertilising.
I don't know if you used Mammoth P on them, as you mentioned it, but that combined with an obviously strong nutrient - mega crop - would be a good recipe for burning the fuck out of your plants.
But you already know most of this, so why not use that knowledge to turn things around?
If you've already given them a good flush, then leave them to dry out before you water them again. Use plain tap water and only water around the base of the plant until the roots start to fill out the medium.
Do not add any more fertiliser or any other additive.
Try to keep your plants out of intense light until they recover. This will slow down transpiration which will also slow down the uptake of nutrient, giving your plants a better chance to recover. Place the plants on the periphery of the light.
Be patient. It may take a week or so for signs of recovery, but you should see they should start to come back if you treat them gently for a while. You may even find they bounce back faster than the new seedlings start to grow, which will not put you as far behind schedule.
But most importantly, try not to make the same mistakes with your new plants. Don't feed them until they need it. Generally, this will not be until the first 4-6 weeks, and only once the roots have filled out their pots.