Why are my leaves yellowing?

Question from a fellow grower:

I need some help ASAP, PLEASE!
have a problem that maybe you will be able to solve. In my grow tent I
m growing 3 Grand daddy Purples. They were growing great and then this
past Sunday I noticed that the leaves were turning gold and drying up.
I moved the 600w Lamp about 18 inches away from them and this did not
stop the yellowing/drying of the leaves.

buds are forming and green and the plants main stems are green but the
leaves are crunchy, I changed the nutriends thinking tha they are not
getting a particular one but no change. The nutrients I am using i
sHumbolt’s own an dI am using the advanced.

the tent is a bag of Carbob Dioxide, a humidifier and a charcoal filter
that blows the air out of the tent. the temperature is 70% and the
humidity is 45 to 50 percent constantly. The plants as of tomorrow will
be in week 3 of flowering. Initally before Sunday the plants were
drinking the nutrients like crazy so much so that I would have to top
the 8 pot hydro they are in up with 5.5 - 6.0 ph tap water on
Wednesdays. Now the plants are barely drinking the nutrients and in
addition to some leaves falling off some of the stems ar efalling off as
well. Again the buds are green, they are maybe a 1/4 an inch and have
what appears to be new leave growth.

Some leaf yellowing is normal because of the plants up-take of nutrients of that specific leaf - it will die . Also you will find that more leaf yellowing will happen in middle and late flower

Sounds to me like you fried your plants with your lamp. You said; “I moved the lamp to 18” away." That is way too close for a 600 watt lamp. How close was the lamp before you moved it back to 18"?

Almost always keep your 600 watt lamp minimum 24-36" away from canopy.