bad plants?

why do my leaves curl under, and why do my first two sprouts turn yellow,why do the second leaves be brown, all in about a week or two,differnt plant?

what's the ph of your soild, how often do you water, whats the ph of your water, what kind of lights, how close are the lights, what size pots are you using, what is the temp of your grow area, and do you have any pics?
you won't get any answers if we don't have all the facts

its in rock wool i water 4 times a day 24/7 lights mh 2550pot well its in a grow box,temp 83 and hum at 47,ph is 5.5-5.8the ppm burned them i think around 500 it jumped to i put it on 150 ppm jumped back up to 500 am useing lemon juice did that raise my ppm?am waiting for my ph down from mail order i got it from advancednutrients i guess its good? i got about a 8 gallon tank its a ebb and flood no pic now put am working on them.

You need to read the GROWFAQ

if you are in soil your ph should be between 6.5 and 6.8 5.5-5.8 is good if you are in hydro...should be in between 50-60 for veg in the 40s for flower...and you should only water once every 3 days or so, whenever your soil is dry an inch or two deep, your soil needs to dry out so oxygen can get to your roots, watering to much can really hamper your plants ability to take in nutrients