HELP UNDERSTANDING LIGHTING

[GARZX9R]

Hi all, could you's help me in understanding lighting, i have a juwel 180 tank with the dual light unit in the hood, i bought the tank second hand and the tubes that are in are T8's but have a yellowish tint to them and just don't show up the colours of my discus at all, could someone explain what kind of tubes i'll need to get a bright clear light, and explain kelvin to me, help will be appreciated
thanks
stu

[ZX10R]

Here is a chart. I think I have heard some people say they like the 6500K lighting the best.

http://i890.photobucket.com/albums/ac101/sig4s/colortemp.jpg