i need help quick

can ne1 help me? my plants are about 4 weeks old now and all of the bottom leaves are turning yellow quickly,my ph is 5.5,im using a hydroponics kit with a 400w grolux lamp,rockwool cubes and i have a fan on so they are not burning , but im using "power-gro" nutrients,the bottle says it has nitogen in it so can ne1 tell me what im doing wrong?am i using the wrong nutrients?

Do the roots have plenty of room?
Cause if the root are confined the whole plant suffers.

are they getting enough nutes? What type of system are you using? Do you give them Magniesium and Calcium? What type of water do you use? Whats the top of the plant temp? What is your NPK?