the leaves are pretty dark green which is why he said overfert..
Yer dude well said, also you got some leaf twisting and cupping real bad on one plant.
Pics 5 and 6 show a quite healthy plant though so all good here and yes it is probably the MG soil. Please be aware that normal potting/all purpose/bat mix/pro whatever mix etc etc are for big plants E.C. starts around 1000ppm and your plants dont look ready for the big numbers yet.
My point is they should basically still be in seedling soil just about ready to be transplanted into the heavier nutrient soils. Go now and buy a bag of seedling soil PLEASE, unless your doing clones this soil is so DAMN important. New growers dont realise this and hence like your plants with no added ferts they are burning and twisting from to much nutes, run now guys and buy that bag of seedling soil, will last forever basically and the best problem solver you ever had for your plants, even on transplant you can mix this in with your normal potting soil to dilute it down a touch.
I put bigger plants than yours into Canna Terra soil and they struggle for a few weeks with the high nutrient level. Peace and happy growing.