Hi Melanie,
The "faulty" bulb shouldn't be faulty. we had it in the light fixture to make sure it worked. You have to really push on the white clip until it clicks in. We ordered them off of Ebay. You are correct with the 250W 20,000K. doesn't matter what brand you use, so long as the connectors on the bulbs are the same.
http://www.ebay.ca/itm/250-Watt-120V...item3a94898cd5
http://www.ebay.ca/itm/250W-250-Watt...item3ce508eb86
http://www.ebay.ca/itm/250-Watts-Met...item19d5dadca7
These three links are the light bulbs that you need. You shouldn't put anything more than 250W in the fixture, But the more Blue you want the higher the K you need, so if you want something more natural you might be looking at 14,000K. The bluer the light the more vibrant the colours.
I would try the faulty light bulb again, When putting it in just make sure its in all the way. Otherwise, I'm really sorry about any conflicting information you've had. I hope your enjoying the tank set up. and please let me know if you have any other questions.