first time grower,need help

I don't even know where to start...Well i'm growing some ww and shiva skunk,they are 3 weeks into veg today,the plants look strong and healthy...I'm using ebb and flow,my room temp is 70-80f humity 50-55%,water temp 68-75f,i have 2 600 watt MH lamps,the lights are 2 1/2 feet away,ph 5.5,i'm using flora nova 1 part grow and floralicious plus at 700 ppms......Leaves were dark green,now some are fading light green..Could it be a nutrient def,could it be cold air,mixing with the warm air,in the grow room...please help! and i'm sorry,for the long post..I have some pics too....thanx alot people