The yellowing leaves on your ZZ plant indicate overwatering. If you're using bagged peat moss based potting mix, than once a week is probably too much water. You should never water on a schedule, but rather only when the plant needs water. I'm not certain what the deal is, but the second picture looks like the leaves are white. I'm guessing it's a photo issue, because I assume that both pictures are of the same plant.

Joe

It's overwatered. The soil isn't draining/drying fast enough. Water less until you can repot in some less water-rententive soil. There's a ton of soil discussion on these boards.

I water my ZZ plant about once a month, and always stick my finger down into the soil to make sure its dry at least a couple inches down. (its in a fairly deep pot, and it takes a while for it to use all the water.) ZZ is much more forgiving of being a little too dry than too wet.

Does this pot have a hole in the bottom?