Abnormal yellowing of leaf tissue is called chlorosis. Yellow leaves on pepper plants may be caused by many factors. Nitrogen deficiency presents as lower leaves turning yellow and then progresses upward. Yellow leaves may also be due to a  secondary nutrient deficiency. Does the fertilizer contain micro nutrients such as manganese, iron, zinc, copper, boron, or Sulfur. Sulfur deficiency would be a more general yellowing. Magnesium would be interveinal yellowing on older leaves. Potassium deficiency will appear as bright yellow leaf edges on otherwise green leaves. Iron deficiency could be yellowing of leaves. High amounts of other elements such as calcium, zinc, manganese, phosphorus, or copper in the soil can make iron unavailable to the plant. Use a complete fertilizer. Manganese or iron deficiency can sometimes be related to root damage. Your pepper leaves may turn yellow if they are under or over watered. Water the pots slowly and completely.  Too much water or poor drainage will cause root damage. Check the roots for root damage.