If your camera has a custom white-balance setting, this should be able to override the green tint you're getting on your photos. You're right, it is because of the mercury vapour lights...
Here's a photo I took of our big guy Zookie using the camera's default "Auto" white balance..

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After I preset a custom white balance and shot again, the difference was very dramatic. The ONLY difference between these two shots is the white balance setting in the digital camera.

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In a 35mm film camera, there are many many different types of film that are calibrated for different types of lighting; Sunny outdoors, incandescent, flourescent, flash, etc. On a digital camera (whether it be a point-n-shoot or a digital SLR), the white balance is essentially digital's answer to shooting with different types of film. Obviously you cant change the CCD or CMOS inside your camera every time you go under different lighting conditions, so this is where the white balance comes into play.
You can spend an absolute fortune getting your white balance perfect. For Digital SLR owners, there's a device called an Expodisc, which is basically just a fancy filter that screws onto the end of your lens. Depending on the diameter of the filter, you can pay anywhere from $80-150 EACH for expodiscs.
For a point-n-shoot, or low-cost alternative for digital SLRs, you can just use a coffee filter. Yup, that's right, the same ones you put coffee into and pour water through to produce a drink 
The way many SLR's work, when you go to set your white balance, you simply put the coffee filter over the end of your lens, point it directly at your primary light source (in this case, the MVB bulb), and fire the shutter. It records that colour light as being white. Then you go back to shooting, and it automatically shifts every photo over so that under that primary lightsource, you get correct colour (although it can still be fooled occasionally, especially if you have combinations of MVB, Incandescent and/or Flourescent all lighting your scene, or animal). This technique also works for quite a few point-n-shoots that have a custom white balance setting.
If your camera doesn't have a custom white balance setting, turn off the MVB temporarily and use your camera's built-in flash. If you find it's too bright, there's probably a setting to knock it down a tad. Some digital cameras use what's called TTL (through-the-lens) flash to gauge the exact amount of light required to light a scene adequately based upon the metering method you give it (spot, center, matrix, or whatever other metering capabilities your camera may have - some point-n-shoots just have a single metering mode). That way the flash never gets powerful enough to blow out your scene. When the sensor detects there's enough light to get a well exposed image, it shuts off the flash.
If your camera's flash doesn't do something like this, you can always dull it down a bit by taping a couple of small pieces of over it. Enough light will still pass through the paper towels to light up your scene a bit, and it will also diffuse the light some to prevent harsh shadows.
Here's an image using TTL flash, but it is unfiltered, you can see the very distinct outline of the shadow under cobra's head - even the tongue is casting a pretty harsh shadow.

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Here's a shot of a Jungle Carpet Python I took at Daytona. This is using just a single flash, with a Lumiquest Mini-Softbox over the flash to diffuse the shadows just a little. As you can see, it helps some, but it's still a little harsh.

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And here's another shot, this time of a 5 month old beardy, in more controlled conditions. This time using 2 flashes. One pointed directly at the camera, diffused through a couple of pieces of paper towel, and a separate flash bounced off the ceiling to further help diffuse the light & shadows. You can see the shadows are much more blurred, and don't draw your eye away from the main subject.

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Depending on your camera though, you could get much better results removing the dragon from his enclosure to a more controlled environment where you have better control of "always-on" lighting (even just regular domes & 60 watt household bulbs will work wonders if they're arranged just right). It may not be as convenient as simply pointing your camera in the tank and firing away, but it should yield much better results. 
I hope this helps you some 
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Axe
The Reptile Rooms