see how much salt build-up there is on those clay pellets? that usually means you are going "salt-toxic".. at that young of age, they do not need much, you are overdoing it.
...personally, I would scrap it because they are so young and start again. They could take 3 weeks to come back..
you should rinse the medium before you try it again but you should choose!. monopotassium phosphate + calcium nitrate + epsom salt should be in every garden and it usually is. watch that bottled crap, they usually have NO IDEA what their shit does to cannabis.
edit.. if you want to fix it, flush your res, add a general 18-18-18 at 1/4 strength. check your ppm, are you using tap water with +200-300ppm?
dude.. try soil before you put seedlings into clay pellets.
Hydro is for pros, dude, get some soil and perlite.!