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Discussion Starter · #1 ·
I have a "throw-away" 5gal acryllic tank that houses some amano shrimp. Unlike my main tank, this one has zero plants. It has a built in hood and a small fluorescent light (I'm pretty sure it is NOT a bulb intended for an aquarium.

A good amount of green spot algae grows on the side with the most light exposure, so I'd like to dim the light output of this bulb. I thought of adding a transparent colored plastic filter inside the hood to dim and color the light. If I do so, can anyone tell me what color would be best to prevent the green spot algae from getting it's preferred wavelengths of light?

As may be obvious I have no interest in putting any money into this setup. Hence this super cheap "solution". Thanks!
 

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The problem with the question is that it assumes that there is a spectrum of light that will minimize green spot algae. As far as I know that isn't the case. I know that having enough phosphates in the water will greatly reduce GSA, in a planted tank, but it should also work for a fish only tank. Keeping nitrates below the level that affects the fish is important with a fish only tank, but I don't believe the level of phosphates that inhibits GSA is anywhere near that which would harm the fish.
 

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Discussion Starter · #4 ·
The problem with the question is that it assumes that there is a spectrum of light that will minimize green spot algae. As far as I know that isn't the case. I know that having enough phosphates in the water will greatly reduce GSA, in a planted tank, but it should also work for a fish only tank. Keeping nitrates below the level that affects the fish is important with a fish only tank, but I don't believe the level of phosphates that inhibits GSA is anywhere near that which would harm the fish.
Thanks, that's a very good point. I suppose I was assuming that GSA 'prefers' certain wavelengths over others.

From things I have read on other forums, and by my own deduction it makes some sense that algae which appears green is reflecting (and therefore not absorbing) green wavelengths of light.

I know this may not be ideal or even valid logic... but humor me: what color filter would I use on a white light so that the only wavelengths it lets pass through are in the green spectrum? Obviously the idea is to only provide green light and 'starve' the algae.
 

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Roscolux - rosco.com Look through the spectrum charts for these filters and pick one that allows very little green through it. It will probably be a magenta filter, for example R4790: CalColor 90 Magenta SED
Thank you very much Hoppy. That is exactly the kind of thing I was looking for - and exceptionally well researched. Your expertise is much appreciated.

Thinking about this before I purchase something... I had posed the question: "what color filter would I use on a white light so that the only wavelengths it lets pass through are in the green spectrum?" But the magenta filter appears to filter out green... not let it pass through.

I'm thinking something like this would be best R2004: Storaro Green SED

A green filter allows green wavelengths to pass through, which are the wavelengths the algae reflects and therefore does not use to grow. -Does that logic appear sound to you? Thanks!
 

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Just because it looks green doesn't mean plants (of any kind) can't use some green..
any blue plus red filter will subtract green..
Magenta in other words
 

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Discussion Starter · #8 ·
Just because it looks green doesn't mean plants (of any kind) can't use some green..
any blue plus red filter will subtract green..
Magenta in other words
See this is where I get very confused with filter colors and wavelengths transmitted... When I look at the data sheet for the magenta filter R4790: CalColor 90 Magenta SED I see that the green-range are the wavelengths being filtered out (less that 10% transmission).

However I believe what I want is for only the green wavelengths to be transmitted. Which is why I'm thinking that a green filter is correct. The data sheet for the green R2004: Storaro Green SED shows that 28% of the green wavelengths are transmitted, and everything else is NOT transmitted (except some red).

Two other posters have already agreed that magenta is the way to go here, so perhaps I'm way off. Then again, perhaps it doesn't matter all that much. I appreciate everyone's input.
 

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@jbjonas you are right , if you want a green light add a green colored filter, this will shift all color towards the green.

The impact on the algae due to the hue of light might be low. I would think that the lowering of the light levels might be more important, as you suggest.

However, just for fun... you could order 2-4 different filters, keep everything constant and see what difference does hue make. Say have a green, magenta, red and blue ND filter. It would be interesting to see if algae die, different algae come or they develop stronger antenna light harvesting complexes.

In either case please come back with the results.

Regards,
duky
 

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Discussion Starter · #12 ·
Just a follow up: Putting in the green filter did help, but I think it's really the fact that it cuts down the overall intensity of the crummy bright white fluorescent bulb, and not that it's filtering any specific wavelengths. Of course, the tank looks absolutely horrible in a shade of green, so we'll probably try a blue or magenta filter with hopefully the same effect of cutting down the light brightness, and maybe it will look nicer. Thanks again everyone.
 

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A way to make this experiment mean more would be to use a PAR meter to make sure the PAR is the same whatever filter you use. You would raise or lower the light fixture to adjust the PAR reading. Now, if the algae growth changes it suggests that it is the spectrum and not the PAR that is causing the changes. Of course this is a much more difficult experiment, and still isn't perfect, since PAR meters that are economical are also not perfect.
 

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Thanks for coming back with the results of the first part @jbjonas . How are the plants doing ? What color are they after switching to non-filtered light output ?

@Hoppy that would be a great experiment. As you mention the response rate of the PAR meters is not a perfect overlap to what is there but good enough for what we do. I was thinking of how the OP or I would approach the light level issue in the absence of a PAR meter.

What about using the light meter in a camera, DSLR for example ?

Keep the camera and light in the same position, same F stop, ISO, WB,same lens length, photograph the same piece of grey paper. Just change filtered vs non filtered and adjust light height until you get the same shutter speed recommendation. Alternatively use a Neutral Density filter that only reduces the light level. Granted this is more error prone than the PAR meter but requires no investment and provides a suggested value. The results of the experiment might not be conclusive but could be suggestive for further testing.

Looking forward to the results of the experiment and to your opinion Hoppy.

Regards,
duky
 
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