most of the retail moonlights are just LED
bulbs glued inside a plastic 'shell'
if you have a nack for DIY
, you can make yourself moonlights for a LOT less than retail.
another thing to consider is "cold cathode" fluorescent. The are typically used to light up the underbody of a car or inside a computer. You can find them at most major discount stores (meijer, walmart) as well as most automotive stores. They vary in price from 15 to 35 depending on size and come in a rainbow of colors.
I currently use two 10" blue-white bulbs on my 29 gallon; mounted inside the hood right above the regular lights. They look very nice in a dark room. I have them timed to come on just before the main lights turn off.
The cold cathode lights run off 12 volts DC
(found in computers and cars), but any "wall wart" power pack that puts out 10 to 14 vdc will run them, just make sure you match up polarity.