help i think my plant is dying

ok my plants have been growing for about 3 weeks and have slowly been getting unhealthy now the leaves on one of my plants is drying out turning brown and dying i think its only one plant and there r 3 others in the same cup i have another cup that has 4 plants in it too and therte all doing ok so far i got some pics here https://www.rollitup.org/members/hashmaster420-199870/albums/flowering-all-new-updates-10005/ can someone point out the problem i think it might be the nitrogen thing but not totally sure

try flushing it with a qaurter strenght nute mix and make shure there is drain holes because your going to soak it. give it three cups water per cup of soil when you flush.
you didnt put them in flowering? you might want to transplant into bigger pots and maybe invest in a better light

yea i am going to move them im acually looking for something bigger to move them to cause i never get anything befor i started growing but i dont have any kind of mixes or fertilizers but i do have more lights i could add but there really isnt any room to put them in the plants r almost touching my full spectrum tube...i am kinda thinking maybe top the one that dying so itll stop growing verticly as fast cause it is my tallest plant and someone said that my plants were like super streched in another post but idk cause iv seen alot of wild weed thats really tall and looks like wat mine is gonna turn out to be...there r some pics here im gonna take some more again cause a couple of my plants look a little better....ooh also i swiched from sink water to bottled sprig water idk if this is helping right now but i kno its healthier for the plants my town water has alot of shit in it.... https://www.rollitup.org/members/hashmaster420-199870/albums/flowering-all-new-updates-10005/