CF and PC are one and the same. They are infact a T5 tube that in effect bent in the middle. Therefore why not just have 2 T5 lineara and choose how to space them rather than be forced to have them 5mm apart???
As per above post kelvin ratings are not important, WPG is inaccurate, lumens is more accurate but going a little over the top really. $15-20 sounds very expensive to me. If you already have the ballast unit and reflector the actual tube should be available somewhere for $8-10. No need to buy one that has the word aquarium printed on the packaging.
Heres a link to show what I mean.
[URL="http/www.theplantedtank.co.uk/lighting.htm"]http/www.theplantedtank.co.uk/lighting.htm[/URL]
Although you are right in stating that lights do diminish over time I think someone has given you some duff info there:
The common thought is (or was with T8 stock ballasts) that you should change your tubes every year. However again this is starting to become an outdated theory.
There are 2 types of ballast unit to power a fluorescent light. Magnetic and electronic. Your typical stock T8s in the hood of your tank when you buy it will be using a magnetic ballast. T5s/PC/CF wether stock or upgrades will typically use electronic.
The difference is that when you turn on the light a magnetic ballast will 'flicker' start' the light. An electronic one will delay 1 second and the light will come straight on with no flicker.
The magnetic ballast will get very hot when used. The electronic one will not get anywhere near as hot.
If you use an electronic ballast the lights will not deteriorate anywhere near as quickly as a magnetic one. This can mean that they will last for 2+ years without diminishing to the point of needing to be replaced. Tubes on magnetic ballasts do noticeably deteriorate over time and I would replace yearly.
I will use my tank as an example again. lol. When I upgraded to PC a long time ago I had to rewire the hood as I wanted to use 1 of the T8s that was in the hood and then fit the PC where the other T8 was. This was because they were wired into a circuit and therefore I had to alter the circuit to run 1 light (remove 1 light from a 2 light joined circuit and the one still fitted wont work as the circuit is broken.) Because I was only using only 1 light I also had to replace the 36W magnetic ballast as it would've overpowered a single 18W tube. I replaced it with an 18W electronic one that I found cheap on ebay. The electronic one actually made the tube brighter due to its greater eficiency. This was 18 months ago and I am still using the same T8 tube. I replaced the PC with the T5HO about 3 months later. Wasn't very impressed with it.
Tubes do not change their K ratings as they deteriorate. They are the same K rating but will just emit less light. As above using an electronic ballast means this is not likely to happen within 2 years let alone 6-8 months.
Algae is not induced by a K rating. It is triggered by ammonia and thrives on plant defficiencies. If the plant defficiency is cause by not enough light then I guess you could say that using a 18000K blue bulb caused the algae but only in a roundabout way. If this were true then noone would use nightlights in a planted tank for the fear of the blue light causing algae. (My tank again. Are you bored of my tank yet. lol)
So in summary:
T5 and PC/CF are both T5, just a T5 is linear and PC/CF is bent in half.
T5 is more efficient than T8.
Electronic extends the tube life.
Sorry for the long post. though I'd explain the whole lot. lol
AC