What type bulbs do you have... T8, T5NO, T5HO or ???
6500K is good for plants as it has red and blue spectrum in it. The higher the K rating the more blue light it has so a 10K bulb is going to make reds look washed out color wise. Also the difference between a lower K rated bulb and a higher K rated bulb is that the higher the K rating the brighter they appear to the human eye. Plants don't see light that way tho. So honestly IMO the 6500K bulb is a good choice. If the tank seems too dull you can always add another single bulb fixture like the one you have so you can add another bulb to brighten the tank up more.
Geisemann bulbs are only T5HO's.