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Problems with Metal Halide spread

3126 Views 10 Replies 4 Participants Last post by  pineapple
I was thinking about my 20 gallon tank problems and maybe the MH is to low. I was looking at these articles to learn more about MH. Most of the stuff confuses me, for example these graphs,

but the other data seems like I would get a better spread by raising the pendants.

I don't know much about how spot lighting works. But my guess is that once the light hits the surface it goes straight down mostly. I think this, because the tank cast shadows and because I am having problems with growing the plants on the edges.
Normal Metal Halide Spread,

Metal Halide at 6" from surface,

Metal Halide at 12" from surface,

You can kind of see what I am talking about in this picture,

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How is the MH bulb oriented in the holder?

____ (length of bulb parallel to the front of the tank)

| or (length of the bulb parallel to the sides of the tank)

I imagine there is a big difference in spread between the two - dependent on the orientation of the reflector.

The 150 watt units suspended above the Amano tanks in Nigata look like they are pretty high above the water surface - about 18 inches I would think.

Andrew Cribb
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parallel to the front of the tank. It is a 70 watt so there is a limit on how high I can raise it. I had it at 16" on the 18 tall and glosso stopped growing. The reflector might not be designed to well on this fixture. I would think getting 2' spread out of a 70 watt would not be an issue.

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You will get much better spread if you turn the bulb around so that the reflector is spreading the light from left to right rather than front to back. This with potentially a little bending here and there of the reflector to focus it correctly should help a lot. Raising the fixture will also help but unless you turn the bulb around, raising it will increase the light that is lost over the front and back of the tank. Being a point-form light source shadows certainly can be strong and an issue, raising the fixture will help as the shadows will be more vertical and will also reflect from the left and right sides of the tank a lot better once the bulb is turned around.

Hope that helps
Giancarlo Podio
The photo of the styrofoam shows the light distribution very well. The sides are dim in comparison to the center. I wonder if it is worth trying out a small reflective sheet to the left and right of the bulb at, say, a 45 degree angle. If you have an old reflector, you can use metal snips to cut out a couple of pieces to try it out. I can't think of another way to better distribute the light apart from turning the bulb around 90 degrees and that is obviously not an option.

Andrew Cribb
Ok, great ideas. I will take a look at it this weekend. Thanks for the help!
That reflector does not look very efficient at all for side lighting. Definitely agree with gpodio on tuning the fixture 90 degrees and seeing how the spread is then.
Place the fixture over a table at the same height, the 'footprint' of the light should be more evident than on the foam. This way you can see where the reflector is sending most of the light and should be able to figure out the best positioning for it this way. You can certainly see on the foam that left to right it's not spreading a lot, even though the foam is a little too close to the light compared to the water or substrate level, but you can't see how far the light is spreading front to back which is where I'm thinking you will have the most coverage.

Giancarlo Podio
Giancarlo, Just to follow up, I think rotating the reflector really helped out. Rather than getting algae on my MM(located on the edges of the tank) midweek the MM has started to grow.
The AQUA MEDIC OCEAN LIGHT 1X70W 10000K is a neat looking fixture. Looking at it, I do wonder if the spread is going to be wide enough.

Andrew Cribb
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