Not too long ago there was a short discussion on the effectiveness (or
lack thereof) of these bulbs. For a recap do a google group search on
"Screw in fluorescent bulbs" One of the points that was brought up was
that the problem may be that the bulbs Richard used were soft whites
instead of 6500K bulbs. Since I have essentially the same setup that he
discussed in his post I decided to try some testing.

Today I went out to Home Depot and bought two 19W 6500K screw in
flourescent bulbs - sadly that seems to be the highest wattage you can
get for a 6500K bulb. What I had in my tank were two 3500K (soft white)
25W screw in flourescents. Since I wanted to see what the difference
would be I removed one of the 25W bulbs and replaced it with the 19W
6500K bulb. It is obviously closer to sunlight since one side of my
tank now has white light and the other has a yellow tint to it (just to
be clear, the side with the 3500K bulb is yellow). However to my eye
the new bulb looks quite a bit dimmer. I guess this is just the
difference between a 25W bulb and a 19W bulb showing up but I'm not
sure which one will be better in the long run.

So rather than going ahead and putting both 19W 6500K bulbs in like I
planned I've decided to leave it half and half for a few days just to
be sure this will be an improvement. For the next few days my tank will
have 19 watts of 6500K light on one side and 25 watts of standard soft
white (3500K) light on the other. As soon as I notice a significant
difference in plant growth (if I do) I'll report the results. I think
it will be interesting to see whether spectrum or total wattage is more
important to plants. Bets anyone?

-Daniel