I agree on the yellowish (= warmer white) of 5500K. The most natural colour rendition of fish (and plants) is achieved under 6000K to 7000K, such as the "daylight" tubes and CFL bulbs rated 6500K. This is also the best plant growing light [I know you said no plants, just mentioning it]. As the Kelvin number increases, the light gets cooler (more blue as noted above).
All fluorescent tubes dim as they age, and rather quickly too. You don't say how long you've had this tube, but the intensity will lessen continually. With live plants this obviously matters a lot, and the tubes need to be replaced every 12-18 months (thinking T8 here, T5 is said to last a bit longer).
One point though, be careful getting bright light with kuhlii loach. These are nocturnal fish, and they will be more out and about in dim light. Floating plants would help here too, and they are easy to grow. I hope you have chunks of wood so the loach can find refuge; a loach with no hiding places will be seriously stressed, leading to health issues.
And last point, you really should have a couple more. I would say no fewer than three kuhlii, though four or five would be much better. Like other loaches, they are highly social fish, evolved to live in groups of dozens.
Byron.
