I work with LED lights all the time, even though the new ones are brighter then the old ones, they are hardly adequate for pleasurable viewing. In my tank, I have an "INDIGLO" blue light. Those flat nightlights that last forever, and only cost a couple cents a year to operate.
If you dig around, you can purchase the raw sheets in all sorts of colors, red, blue, green, aqua, violet, white, yellow, orange. Do a search for Lime-Wire, or EL-Wire. The material is called ElectroLuminesent Film Thus, EL-Wire. If you need it brighter, get the "WIRE" instead of the "SHEET". You can also search the net for "CASE MODS" refering to computer case modding communities. They also sell the wire at auto stores like "DISCOUNT AUTO", "NAPA" etc... and at car stereo stores such as "WIRED", "LONG RADIO"...
These are surface emination devices, similar to how a flouressent light eminates from every gas particle, except this is really more like a "GLOW BLOCK" on steroids. There is a real shallow decay range of the eminated light, like a flouressent bulb, only shorter.
Things to bear in mind, these operate on high voltage, 220v - 110v at a real high frequency, frequency determines brightness, not volatge... and also determines the halflife time. They will last forever if moisture doesn't penetrate the plastic coating. Although they last forever, they have a half life which becomes exponintially longer with age. EG... after two years this will be 3/4 as bright, after four years it will be 1/2 as bright, after eight years it will be 1/4 as bright... etc...
This will light the tank enough to see inside, and turn the inside glass into a one way mirror, so they can't see you that well. Thus aiding in your snooping endevors.
Get creative, buy the DIY kit with one 4" sheet, and cut it out into a full moon, go so far as to draw little craters, or if your printer can handle printing on cardboard, print a black and white moon right on it and cut it out to the shape. Just make sure you seal all the edges with fishtank caulk, as the cut edge will be the first to be exposed to moisture... Then stick it on the top of your tank cover, inside, or place it on the outside of your back wall of the tank, between the background and the glass. Have it on a timer like your lights are on. Get even more creative and hook up a PHOTO SENSOR or DAY SENSOR so it turns on when the lights go out.
Instead of using a sheet, use the wire and tuck it under the black plastic fishtank trim, just out of sight. If you get real creative, you can etch a design on the glass and it will glow as if it floating in the middle of no-where. The glass will carry the light to the etched part and release the light where the glass is etched, lighting it up like a breaklight.