Yellowing leaves occur when plants are drowning, dying from drought, starving and other plants. You did not give me enough information to determine if you are over watering. Geraniums in a pot need more water than those growing in the garden. Do not water based on a set schedule. Water based on the plants needs. let soil dry slightly between waterings. Check potted plants daily (sometimes more if in sun and it is really hot). Use your finger to feel soil. Work your finger into the soil an inch or more. Or use a freshly sharpened pencil. Shove pencil in about three inches an inch or more from pot rim. Remove and check point. If soaked or dam do not water. If dusty - water till water comes out the bottom of pot. If water comes out right away, let pot sit in water for a few minutes, then water again and pour off excess water that comes out the bottom. If you have to water a potted geranium a lot, you leach the nutrients out with the watering. Use a house plant fertilizer at about 1/10th rate with every other watering.