Does it have two tubes and a manual "push to start" button? If so, you can usually get it to light up only one tube instead of both of them, depending on how you push the button, as seen towards the end of this video:
It actually has two buttons, one for each light, but each light is still too bright, even by itself. I saw a fish tank bulb in Wal-Mart that was about half the brightness of one of their brightest bulbs, but I didn't know if that would be low enough. They also have blacklights in this size, but it doesn't seem like that would be too helpful.
Fluorescent tubes work by creating UV light, which itself isn't all that useful to use (actually, it can be pretty nasty) so the inside of the tube is coated with a phosphor that illuminates strongly in the presence of UV light.
Just in case you didn't know that already.
Anyways, there is not really much you can do to "reduce" the brightness of a tube short of say, wrapping the tubes in sheets of paper. If you drop the voltage to/from the ballast eventually the ionized cloud inside cannot be sustained and it simply turns off. Your best bet is to find tubes with a lower wattage rating.
The bulb I removed has "FL-15D Daylight" written on it. It's an 18" bulb.
I'm trying the aquarium bulb, and it's a little dimmer, and the pink light is less harsh, but it's still not ideal. I've been using a piano light with a frosted incandescent bulb and it was the right brightness, but I wanted to change the style of my lamp.