This thread has been flooded with crummy advice. Bud you honestly appear to have a nitrogen deficiency... This could be caused by not enough nutrients, or (most likely) your pH is off... When your pH isn't at the appropriate level (Which it probably isnt because those test kits are crap, you need a REAL meter) the plant cannot uptake certain nutrients... If your plant is unable to process N, it will begin to yellow, it wont die off immediately, but it wont be green (which it needs to be), and growth will be slow...
Check your pH (With a numbered meter), get us the results back ASAP... and then I'll help you fix this...
thread subscribed for now.....