hydro help

hey i just started to get into hydro and i got a sh kit. for now im experimenting with just plain tomatos to get a hang of hydro so far ive killed about 3 plants and one looks sick has yellow on it but 2 are doing what appears the be just fine here are some pictures i was wondering if anyone would be able to tell what im doing wrong.

i have them in artifical light 24hrs a day in a closet with a fan going the tempeture never exceeds 80 degrees its been two weeks so far and i havent changed the grow stuff yet