Why do people suggest reducing intensity of [insert color here] to stop algae?

[damsels are not mean]

On just about any thread about algae outbreaks, especially from beginners or new tanks, there is always someone or a number of people who come in and suggest reducing white light, or green light, or red light, or whatever else. Back before LEDs were popular it was "change the bulbs, old ones grow algae!".

Is there anything (like some real studies or even just work threads) to back up the [insert color] grows algae or is this just like the bulb thing? Obviously less light = less growth but this also means less for the corals...

 

[ISpeakForTheSeas]

From what I've read, it's because algae generally uses different light wavelengths for photosynthesis than coral zooxanthellae do - corals using more blue light, and algae using more red and green/yellow.
For a simple explanation, here's a graph from Khan Academy about photosynthesis and light absorption:

I wish I could find the graphs that I've seen specifically for corals/algae, but I can't remember where I've seen them. Anyway, basically the "Windex" blue light tanks use that spectrum (and some people recommend it over using white/red lights) because - at least theoretically - the corals do fine using primarily the Chlorophyll A provided by the deep blue while essentially starving out the algae by not providing the green/yellow and red lights for Chlorophyll B and Beta Carotene. I've seen some more complicated charts of this with more absorbing pigments than just these three (Chlorophyll C, Chlorophyll D, PSC, and PPC to name a few), but - again - I can't find the ones I've seen for corals which would be more useful, and this one gets the point across.

So, the theory is that you feed the corals with blue light, and the algae with other lights. By reducing the other lights, you stop feeding the algae/give the corals the competitive edge. That's why people say to reduce intensity of other colors to stop algae. How accurate that theory is would likely depend on the specific corals and algae involved, but the theory seems relatively sound when speaking broadly. Since many (possibly most) algae species utilize Chlorophyll A, I have no idea if it's effective or not, but there is a chance that it gives corals the edge they need to beat out algae. I'm not aware of any studies that actually show one way or the other though.

With regards to the "... old ones grow algae!" - here's a quote from Dana Riddle on it that basically says it's possible following the same lines of reasoning listed above, but he's not (or at least wasn't) sure if it was true:

I hope others will chime in here - It has been a axiom in the hobby for years that degradation of spectral qualities in certain lamps (fluorescents, metal halides) will encourage certain algae to grow. The reasoning is usually that the lamp becomes 'more red' (suggesting the total amount of red increases) which gives me some heartburn - the lamps often becomes less blue and the percentage of red increases. The difficulty (for me at least) is separating intensity from spectrum. It is true that chromatic adaptation can occur, but I'm not at all sure preferential growths will occur. My focus for years has been responses of zooxanthellae to light intensity and how it might affect coral growth, so responses of green, brown, and red algae are a bit out of my field (although there is no doubt that their absorbance qualities differ.) Anyone - ideas, suggestions?

In case you're curious, here's a graph from the paper linked below that shows different classes of algae use different spectrums of light for their photosynthesis:

https://www.researchgate.net/publication/333210678_Algal_Biofuels_Current_Status_and_Key_Challenges

 

[A_Blind_Reefer]

On just about any thread about algae outbreaks, especially from beginners or new tanks, there is always someone or a number of people who come in and suggest reducing white light, or green light, or red light, or whatever else. Back before LEDs were popular it was "change the bulbs, old ones grow algae!".

Is there anything (like some real studies or even just work threads) to back up the [insert color] grows algae or is this just like the bulb thing? Obviously less light = less growth but this also means less for the corals...

I understand the logic but don’t understand why you would want to aggravate coral by changing the light spectrum or intensity. I have noticed this as well. Someone has a relatively new tank, just dumped some money of corals and has a gha outbreak. Dim your lights, turn off the white channels, blues only. Dump 20,000 snails in there. I’m no expert, but how about pull the algae out and figure out what nutrients are out of whack so you can start to fix the real issue. Opinions are fine, but there’s a lot of fact throwing around here. If you see someone has all kinds of tags under their name, you may be inclined to believe them.

 

[damsels are not mean]

From what I've read, it's because algae generally uses different light wavelengths for photosynthesis than coral zooxanthellae do - corals using more blue light, and algae using more red and green/yellow.
For a simple explanation, here's a graph from Khan Academy about photosynthesis and light absorption:

I wish I could find the graphs that I've seen specifically for corals/algae, but I can't remember where I've seen them. Anyway, basically the "Windex" blue light tanks use that spectrum (and some people recommend it over using white/red lights) because - at least theoretically - the corals do fine using primarily the Chlorophyll A provided by the deep blue while essentially starving out the algae by not providing the green/yellow and red lights for Chlorophyll B and Beta Carotene. I've seen some more complicated charts of this with more absorbing pigments than just these three (Chlorophyll C, Chlorophyll D, PSC, and PPC to name a few), but - again - I can't find the ones I've seen for corals which would be more useful, and this one gets the point across.

So, the theory is that you feed the corals with blue light, and the algae with other lights. By reducing the other lights, you stop feeding the algae/give the corals the competitive edge. That's why people say to reduce intensity of other colors to stop algae. How accurate that theory is would likely depend on the specific corals and algae involved, but the theory seems relatively sound when speaking broadly. Since many (possibly most) algae species utilize Chlorophyll A, I have no idea if it's effective or not, but there is a chance that it gives corals the edge they need to beat out algae. I'm not aware of any studies that actually show one way or the other though.

With regards to the "... old ones grow algae!" - here's a quote from Dana Riddle on it that basically says it's possible following the same lines of reasoning listed above, but he's not (or at least wasn't) sure if it was true:

In case you're curious, here's a graph from the paper linked below that shows different classes of algae use different spectrums of light for their photosynthesis:

https://www.researchgate.net/publication/333210678_Algal_Biofuels_Current_Status_and_Key_Challenges

Makes sense, but seems like many of the ones listed there use the same chl A and C that corals do with only a couple using B, no?

 

[ISpeakForTheSeas]

Makes sense, but seems like many of the ones listed there use the same chl A and C that corals do with only a couple using B, no?

Yeah. That’s why I said that it’s possible it gives corals the edge, but - personally speaking - I doubt it’s a big enough edge to make a meaningful difference for most algae (and, personally, it’s not something I’d “Windex” a tank for). That’s why people suggest it though. Personally, I’d say that if you’re going to turn your tank blue, you should do it for the fluorescence, not for algae control, but that’s just me.

 

[ZombieEngineer]

Honestly, I think it's carry over from the old metal halide days. Unless you were previously running waaay too much whites or have a tank next to a window, turning whites down will do very little to solve the actual problem.

The only time I would recommend people cut whites is for something a bit more serious like dinos. Otherwise, people should focus on the root problem.

 

[laezur]

From my own personal experience, when I unfortunately had a fish death and left my tank with only corals I turned down whites completely for two weeks, and all my hair algae is now gone

 

[ZombieEngineer]

From my own personal experience, when I unfortunately had a fish death and left my tank with only corals I turned down whites completely for two weeks, and all my hair algae is now gone

You also probably stopped feeding for two weeks. This is more likely the cure than turning off the whites. The whites had some effect for sure, but probably the lesser part.

 

[Hairyteeth]

From my own personal experience, when I unfortunately had a fish death and left my tank with only corals I turned down whites completely for two weeks, and all my hair algae is now gone

And your main source of nutrients was also removed….

 

[laezur]

And your main source of nutrients was also removed….

Totally, although my feeding was very light for two clownfish. More likely the nutrient control but I always had very good nutrient export

 

[stacksoner]

Look at Chaeto reactors or Algae scrubbers. They both specifically and exclusively
use these colors to grow macroalgae.

 

[djf91]

You see that recommended a lot about turning up blues and turning down whites, but I think it’s just anecdotal. My Radium MH bulbs are nearing the end of their life right now and my GHA problem is just now going away…..so I don’t buy the whole “old bulbs grow algae” thing. What’s likely happened is my tank and live rock have finally matured and outcompeted the nuisance surface occupiers.