Need Help on Diagnosis (Heat? Light? Soil?)

Those plants are in there for WEEKS already, and I am talking about 3-5 weeks. And they look awful.
Two of them in the back of my greenhouse are yellow, almost white and they grow extremely slow.

I have 300W MarsHydro LEDs (pic below) above them and I already moved the lights back to 30" since so many said that seedlings are very sensitive and my suspicion always was it's light bleaching.

The soil we use is a 6.0ph universal "cheap" soil which however is supposed to be "good", it's sold here in Spain in supermarkets and I looked up the brand in Spanish grower's forums and people in general said this is an ok/good soil for MJ.

The problem is the currently IMMENSE heat and I don't really know what to do. I am growing in a greenhouse on our balcony, the plants do only get little direct sun (in the morning) but the greenhouse is in the shade the rest of the day. This the reason for the LEDs. So basically, regarding the lighting you have to add that I get normal daylight and consider that the LEDs are not the only light source.

The heat right now is murderous. I am talking 95F-100F (36C-38C) at noon, 87F at night. I cut large vents in the greenhouse, massive openings on the side, openings in the roof etc. so that I would say air flow/ventilation in the greenhouse is "ok". With the lights on it's only 2C warmer inside the greenhouse as compared to a corner on the balcony. (Mind you again that the entire balcony is pretty shaded, there is even a roof over the area).

Since the lights are already at 30" (which is ridiculous far seeing that the recommended distance is 18"-24") I am more tending to say the problem is heat stress?

We are entirely new to growing...but seeing how awful those plants are doing makes me sad.

PS..there are some more pots in the greenhouse and the plants there look fine and nicely green, it's only those two in the back which are pale yellow.

Any ideas what I can do?

Thanks!

Edit: Ok I am just speculating here, but maybe there is a hotspot in that corner of the green house..so today/tomorrow I will put the thermometer actually right there where those pots are...it's a possibility that airflow right there is not "optimal". Or it's simple that strain which has problems with LEDs since the other plants, different strains, look fine?

Heat probably has something to do with why its so slow but it could also be a problem with over watering, I mean I guess a lot is evaporating but still it looks like you gave it a decent amount of water, maybe im the weird one though because I probably feed my seedlings 10-15 ml of water a day with a pipette (but I also start my seedlings in solo cups so youd definitely need some more with your pots) until they out grow the cotyledon leaves, thats probably the low end of what youd get away with but yeah just something to try although mind you dont let it totally dry out or theyll die pretty fast at that size.

i have plants that grew at shitty heat and humidity, seedlings that don't grow usually have a bad case of shitty soil conditions. the seedling stage and the flowering stage are the ones you need to really study on to avoid these problems

i had one plant at around 30c on average and was watered once the moment it sprouted from soil and i actually left it for about 10 days for a vacation and when i came back it actually grew decently without any deficiency or problem. (led light at around 24')

keep it basic at the start is all i can say. you won't even need a fan till it gets a few inches taller.

and use cfl for little ones for about 1-1.5k weeks till they grow a few sets of leaves so that they'll make use of the 300w LED.

also probably the genetics of the seed would help in certain conditions/temps/humidity/whatever

i use sunlight and cfls.. 6 cfl lights 3x2500K and 3x6000k...23 watt each...my plants seriously get 1.5 centimeters every damn day...so i really wanna know how much are they gonna get when i start flowering...