Put both on the tank for now. Unless it's not too late to take the fixture back?
I kind of doubt the 18000k bulb will support plant life but it depends on the manufacturer; some are OK for FW plants. In the future I'd search for a fixture with bulbs between 5000-10000k, though.
You have lots of options as far as fixtures. The Glo fixture you have now wouldn't be bad with 2 bulbs instead of just the one, and if that bulb is the one with red peaks in the spectrum (I recently learned that there's 2 18k bulbs- one is just for SW corals but the other is OK for FW plants)
Shoot for something that gives you between 60-90 watts, total- then you'll make more use from your CO2. You'll also need to use ferts at that level of lighting. Seachem makes good liquid ferts, or you could buy dry ferts (much cheaper in the long run) from
www.RexGrigg.com There's tons of good info on that site as well- you might want to spend some time there reading up?