plants look bad: yellow and brown leaves. help!!!!!

sorry fellas i dont have the means of posting pics. this is my first time growing and i'm about three weeks in. im using a hydroponic drip system six plants to a tub. more than half of my plants are starting to turn either yellow or brown on the leaves and some of the leaves are starting to curl up like tacos. im running a 400w hps lighting. please help i want to save my babies!!! two days ago i dumped all the water and changed it. now im not running any nutes just 5.8 ph water. was that the right thing to do? or not please help its getting bad and i dont want them to die, is there any saving them?

What brand of nuits.? what is the nuit. temp?

Do you have a Ph/PPM meter? what was the reading?

Are you running any airstones, where?

how often do you feed ?, during the dark cycle?

Raise the light ,stay on Phed H2o for a while

time will tell

read, read , and read some more!