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DIY - Cold Cathode Moonlighting
by Robert Viau


Browsing through the forum here, I'd seen more than a few posts that talked about using a blue light source to create a "moonlight effect" in the aquarium. The idea of adding this artificial "moonlight" to my aquarium intrigued me for several reasons: the aesthetic value, the additional viewing time it would offer, and the fact that it would help to more smoothly transition from light to dark. After reading as many posts as I could find on the topic, I found out that there were a number of different ways to do moonlighting, the two most popular being cold cathode tubes and light emitting diodes or LEDs. In addition to these two methods, there was a number of other ways that people were creatively adding homemade moonlight to their tanks. I saw everything from blue Christmas lights in the canopy to those flat blue nightlights taped to the outside of the glass. I started to get the idea that some people were shying away from the mainstream methods of cold cathode or LED moonlights because they thought they might be too complicated to hook up. Hopefully this article can help to dispel that myth-about cold cathode lights. In my opinion, cold cathode lights should be easier to set up than LEDs because less knowledge of electronics is necessary and, best of all, no soldering! Recently I've seen LED "kits" that make them easier to install, but it is also my opinion that cold cathode lighting provides a more natural moonlighting effect. In fact, users of LEDs often complain of bright spots in the aquarium directly beneath each LED, while the use of cold cathode tubes allows you to spread the light evenly across the tank. For these very reasons, I decided to go with cold cathode tubes to create the moonlight effect in my aquarium. Here's how I did it...

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The complete cold cathode kit, including tubes, inverter box, and wiring harness.


I have an All-Glass Twin Tube strip light, but the installation should be similar for other lights as well. In order to make things simple, I've broken down the installation into distinct steps. You should be able to complete this project in about an hour.

Step 1: Preparation

Unplug the light strip and flip it over on a sturdy work surface away from the aquarium. Before you remove the fluorescent bulbs, it is a good idea to test fit the cold cathode tubes. Place the tubes on the reflector and move them around until you find a good position that allows them to be centered in the fixture but does not conflict with the main fluorescent bulbs. Once you get the cold cathode tubes set, mark their location on the reflector. This will prevent any problems when it comes time to put the fluorescent bulbs back in. After marking the location of the cold cathode tubes, set them aside and remove the fluorescent bulbs as well.

Step 2: Reflector

Once you mark the location of the tubes you need to remove the reflector to gain access to the inside of the light. The All-Glass light strip has plastic clips on each side the hold the reflector in. I carefully pinched the clips with a pair of pliers to remove them and moved the reflector up out of the chassis. Be careful with this step as the reflector is connected to the chassis by the wires that provide power to the fluorescent bulbs. You just need to move the reflector enough to gain access to the inside of the fixture without putting any stress on the wires inside. Next, drill a hole in the center of the reflector to route the wires for the tubes. This can be any size hole as long as it is large enough for connectors on the ends of the cold cathode tubes to pass through. WARNING: Make sure that there is nothing (wires, ballast) behind the reflector where you are drilling the hole! Once the hole is drilled, insert a store-bought rubber grommet or carefully line the hole with electrical tape as the edges will be quite sharp and could cut the wires if left unprotected.

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The light strip with the reflector removed and the first hole drilled.

Step 3: Chasis

The next step is to drill another hole in the back of the plastic chassis to route the wires back out of the light strip. Make sure that this hole will clear the reflector when it is put back in the light. There is an area of empty space between the top of the light strip and the reflector. The hole needs to be drilled in this space, above the reflector. After the hole is drilled, run the wires out and connect them to the inverter box.

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An example of how the wires may be routed and the location of the holes.

Step 4: Cleaning & Mounting

Next, put the reflector back in the fixture and insert the clips to hold it in place. Clean and polish the reflector to remove any fingerprints, dirt, etc. that might be on the reflector as a result of the installation process. This is important for two reasons-you want the cold cathode tubes to have a clean surface to adhere to and the reflector should be as shiny as possible to reflect the most amount of light back into the aquarium. Once the reflector is clean, line up the cathode tubes with the marks that you made earlier, and attach them to the reflector using double-sided mounting tape. The mounting tape that is included with the kit looked a little flimsy to me so I used a roll of heavier mounting tape that I had lying around. Once the tubes are securely mounted, push the excess wiring back inside the fixture and mount the inverter box on the back of the chassis using the same mounting tape.

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A close-up of the wiring with the lights installed, mounted and running.

Step 5: Power Supply

Because these cold cathode lights are designed for computers, they are set up to plug into a PC power supply. I tried this at first using an old power supply that I had lying around, but it didn't take me too long to realize that it wasn't going to work out. First, the fan on the computer power supply made too much noise under my stand. Second, the light was way too bright--I was looking for something that would highlight the rocks, but when I first turned my lights on they flooded the entire tank with such an intense light that even the sand looked bright blue. I had read about others using AC adaptors to power their lights so I decided to give it a shot. WARNING: Make sure that you use the proper AC adaptor for this project: one that converts AC to DC. The adaptor should say "DC" near where it lists the output voltage. Some AC adaptors are actually putting out AC voltage, which will burn out the inverter box. I tried several different AC adaptors with mixed results. One thing that I noticed is the fact that the amps are much more important than the volts when it comes to the light intensity of the tubes. Here are the results of my testing:

12V DC8.0A Way too bright (PC power supply)
9V DC1.0A Pretty good, not overly bright
9V DC0.5A Tubes would not light
6V DC0.4A Tubes would not light
5V DC0.8A Tubes only lit halfway, too dim

I decided to go with the 9V, 1A adaptor because the light output was pretty close to what I was looking for, and I was tired of cutting up AC adaptors at that point. Hooking it up was relatively easy because the wiring harness attaches to the inverter box with a simple snap connector. It is a good idea to separate the harness from the inverter box so you don't accidentally cut the wrong wires. All modifications are to be made on the wiring harness itself. I cut the wires between the connector and the switch on the wiring harness and spliced the connector to the AC adaptor. If you get these wires mixed up, the tubes will not light up. Simply reverse the wires and try again. The leftover portion of the wiring harness (with the switch and the Molex connectors) is not needed. At this point you should have an AC adapter with a snap connector spliced on the end. Connect this to the lead coming from the inverter box and you are ready to go. This connector also makes it easy to disconnect the AC adaptor when the light strip needs to be removed from the aquarium for tank cleaning, water changes, etc.

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The AC adaptor with the connector spliced on.


Finally, put the light strip back on the aquarium and plug the AC adaptor into a separate timer. I set mine to turn on a little before the main lights turn off and to run until about 1 a.m. The way that the tank looks under the blue lights is exactly what I was looking for and this project turned out to be a lot easier than I thought it would be. The best part is that I was able to do it all myself, and I think you can too!

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Two photos of how the aquarium looks under the "moonlight"
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Disclaimer: By building this DIY project you agree not to hold the author or the owners of this Web site responsible for any injury or bodily harm you may cause to yourself or others. Always wear safety glasses when working with tools and keep chemicals and power tools away from children. Read and understand all safety instructions pertaining to equipment prior to use.
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