Brown spots on 3 week veg plants. HELP

One plant developed brown spots a couple days after transtplanting. I waited about a week (the plants are now 3 weeks old) and gave them their first 1/4 strength dose of grow juice. Now i have 2 other plants beginning to develop the same brown spots. What could it be and what should i do to fix it?

Started plants from seed
Growing medium is Promix-hp with 6 to 1 worm castings.
1000 MH bulb about 36 inches above the plants
temp ranges from 68-80 degrees in direct light
Single feeding of 1/4 strength grow juice

maybe feed some more
what's the water?

mg def maybe not sure check the defeciancy chart thingy

there
https://www.rollitup.org/attachments/marijuana-plant-problems/1956527d1317184266t-what-leaf-chart.jpg

iz small i know cant find da big one br() x)