3 Week Old Seedlings,Older Leaves Have Brown Spots,Yellowing,Cotyledons Not Dead

Hi everyone. I have a few seedlings that sprouted three weeks ago that are under an 8 bulb t5. I have a temp probe at canopy level and the temps are 77-83 ~50% humidity. A few of the seedlings are turning a yellow shade and the lower leaves are getting brown spots on them. I have been feeding them 100ppm cal-mag in distilled with 200ppm nutrients. For some reason none of the cotyledons aren't dying yet so i'm a little hesitant to add more nutrients. the pH is 5.7 and i calibrate my meter each time. the cal mag is over a year old and it sort of looks like Mg++ deficiency. i've had spider mites in the past and this is definately not a bug problem. i dont think that its heat stress either since the spots are only on the lower leaves. the other seedlings that i have that are dark green are drooping slightly while the yellow leaves seem to be fanning up towards the light.

the ph of soil shood b at 7. , 5.7 is hydro ph u may have lock out , ph the run off if its close to the 5.7 mark thats the problem if its 6.8 -7.2 theres other things happenen . if indeed the ph is off flush with clean water , imo rain water is best 4 soil and given the storms lately that should not b a problem 2 come by if it is distilled works well .

they are in coco runoff is 5.9

its not straight coco is it ? it looks 2 have other material worm castings maybe , still 5.9 too low needs flushing when it dries up a little

at 200 ppm it wood still b hungry . i missed that bit of info as well .

Im have problems with my plant need help brown spots on leaves here r pic of it

sure looks like dirt to me