# How long to leave the light on?



## BSS

Hmmmm. Not completely sure. Many recommend 10 hours for a high light tank, which is what I have in my 46g. But, my low-tech 10g probably has the lights on for more like 12, and it doesn't seem to bother the setup. It's full of anubia, fern, crypts and the like.

I guess so long as things are growing and algae isn't a problem, let it go a bit longer.


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## Raul-7

Plants usually stop photosynthesizing after about 8-10 hours; there's no need to leave it on for 12 or more. Remember lighting duration does not make up for lighting intensity; just leave it on for 10 and your plants will be fine. Anything longer than that is just giving the algae the edge.


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## snafu

Raul-7 said:


> Plants usually stop photosynthesizing after about 8-10 hours


that's a bold statement. can you point us to some references on this? what are the mechanisms inhibiting photosynthesis after 8 hours? since i'm rather ignorant on plant physiology, keeping the explanation simple (in layman's) terms would be nice and would probably be helpful for the community. thanx
-snafu


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## JenThePlantGeek

Raul-7 said:


> Remember lighting duration does not make up for lighting intensity...


Well, some experts say otherwise. In Aquarium Plants by Christel Kasselmann, there's ONE sentence that says that lower light intensity can be supplemented by extending it's duration. She didn't explain further. It's debated pretty hotly too, but at least one SUPER GREEN thumb says that longer light intensities are good. Kassellmann gets most of her evidence for this using a light meter and studying the natural habitats of the plants she collects. She's been all over the world (something like 25 trips for collecting plants?) so I'm guessing she knows what she's talking about. 

It seems like no matter what you have, a 12 hour photoperiod does well. This is what Kasselmann recommends too, and I personally have used 12 hours of light with great success on both low tech / low light and high tech / high light / CO2 setups.


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## Raul-7

Look at Cabomba and Rotala species, the leaves close together after about 10-12 hours regardless of how long the light is on. And I've even seen P. stellata do this as well. I remember reading an article on photosynthesis on APC, but I can't find it now.


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## Hoppy

My two cents worth: a 12 hours on, 12 hours off cycle can't be far from the natural light cycle, so it should work fine. Any longer light duration isn't likely to benefit the plants. But, using twice the hours for half the light intensity doesn't work.
I suggest you look at the AH Supply website, http://www.ahsupply.com/ and see if it changes your mind about retrofitting the lights. At worst it will frustrate you if you can't afford to do it. At best you will go that way and have a nicer looking pair of tanks with more flexibility in what plants you can grow.


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## snafu

ok, now we've up'd the ante so that 12 hours 'might' be acceptable. that still doesn't address the question of the mechanism at play that makes any incremental time unbeneficial to plants. naturally, i can point to many sources saying otherwise, as extended photoperiods are used all the time in other industries for horticulture, floraculture, etc. of course they have different objectives of growing seedlings to sellable product in the shortest time.

the other statement is also counter-intuitive to me as well. half the light at double the duration clearly does NOT make sense. i wish all things in life were linear like that. heh (it would make estimation a lot simpler.) but extending light duration at lower light intensity seems logical to a certain point, as long as the intensities are above the low light compensation point. it seems like once you have excess energy that can be applied for plant growth, increasing duration seems beneficial. of course the plants will adapt to low light conditions and the leaf structure or stems will change to its environment (possibly in an un-aesethically pleasing way. heh).

can someone help a newbie better understand this? for the record, i keep my photoperiod below 12 hrs. thanx.
-snafu


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## rrguymon

Hahah. I lived in Alaska a few years. In the summer the days are long with lots of sunlight. You should see the size of the garden vegatables they can grow in that extra light. I am not saying this will apply to our tanks. I don't think it does in most cases. 

Rick


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## nik

This is way too complicated to explain easily here. Just keep in mind that there are both Photosystem I, which is light dependant, and photosystem II, which is not. There are also different kind of plants, like C3, C4 and CAM. CAM plants are depentant on fixing CO2 at night, because the vacuole is closed during the day.

For those interested enough I'd suggest reading up on some of these terms. For everybody else it should be fine to use the tried methods (10-14 hours?) or choosing periods that correspond to the length of day where the plants are from... Of course, that could be hard. Here in northern Norway we have 24 hour of sun in the summer...


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## Betowess

Well Buck has said anything over 10 hours just helps the algae grow. But I leave mine on whenever I want to look at the tank, so sometimes its over 12 hours.


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## railstar

Thanks for the great discussions and information. It seems like 12 hours seems to be a good place to start and I'll try that out and see how that works.

I have considered retrofitting with AH Supply kits and I spoke with a representative at AH Supply and he said he honestly does not recommend their kits on the Eclipse systems because of the inability to keep moisture out of the lighting fixtures and heat issues where sometimes the plastic hood will melt or bow. Ack.

But, let's say I did retrofit with the AH Supply kits- 1x13 for the 6 gallon and 2x13 for the 12- would I need to investigate the need for CO2 due to the increase? I'm trying not to get myself involved in CO2 for various reasons right now.


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## Hoppy

High light almost demands that you use CO2 injection. The intensity of the lighting you use pretty well determines all of the rest of the routine - the fertilizing, the CO2, the water changes, etc. You can use Seachem Excel to improve a non-CO2 situation, but it still isn't equivalent to injecting CO2. In any aquarium you should try to make the light be the only limit on how fast the plants grow - always have enough of all of the fertilizers, including CO2, so only the light restricts the speed of plant growth. That isn't at all difficult with low light tanks.


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## plantbrain

8-10 hours is about what most natural systems get underwater.
You can see most fine needeled submersed plants, like Cabomba, Rotala wallichii, Myriophyllums, you name them, all close up after 7-10 hours of light.

So there you go, why on earth would they close up if they "wanted" to keep photosynthesizing?

It's not just one species nor one genus that does this, it's a definitely pattern.

Are plants trying to fool us?
I don't think so..............

Some species certainly will PS for more than this, but this is all that's needed as a rule, 10 hours works fine. Kasselmann also makes a very bold and quite incorrect comment that 10 hours will kill her plants.

Anyone on the web knows this to be patently untrue.
She does not offer any referenced support for her contention either nor do we know what the other piotentially confounding factors lead her to claim such things either.

I do know that the statem,ent is wrong when I repeat the claim testing the light and have no issues.

For her statement to be true, we'd have to see lots of folks have melted weeds withg 10 hours of light.

Clearly we do not see that, therefore she needs to revise that hypothesis because it's rejected.

Nothing wrong with being incorrect, I make many mistakes before I get one thing right, but that's why I know when I am right, I've already made the same or similar mistakes as other folks and if I do not know about something or if it really is not significant, I'll say so.

Regards, 
Tom Barr


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## Hoppy

> "Are plants trying to fool us?
> I don't think so.............."



It's a vast aquatic conspiracy!


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## snafu

plantbrain said:


> 8-10 hours is about what most natural systems get underwater.


generally true, unless you're at high latitudes.



plantbrain said:


> You can see most fine needeled submersed plants, like Cabomba, Rotala wallichii, Myriophyllums, you name them, all close up after 7-10 hours of light.
> 
> So there you go, why on earth would they close up if they "wanted" to keep photosynthesizing?


interesting observation... the 'closing up' of fine leaved plants is completely independent of light intensity, spectrum, and length of non-illumination periods?



plantbrain said:


> Kasselmann also makes a very bold and quite incorrect comment that 10 hours will kill her plants. <stuff deleted>


not having read any Kasselmann, it's unclear (based on the two msgs in this thread) whether she's saying 10 hours or less of light is bad (i'm assuming this is the case)? there is a distinct difference between the lighting in natural and artificial environments based on the temporal variation of incident flux. in our artificial tanks, we generally have high, constant lighting conditions. in the natural environment, the light follows a more parabolic curve peaking at solar noon. so, given equivalent peak solar irradiance, 10 hours of natural light might provide significantly less energy than a similarly lit tank. basically, we might be comparing apples and oranges. maybe this is where she is coming from?


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## MIL007

I enjoy this thread - 

That said - my take in terms of a hobbyists understanding is that the best light profile is the one that does not stress out your inhabitants or grow excessive algae.

I can find some agreement with most the folks who posted. In a sense - I say THERE IS NO specific ANSWER in our hobby - because most of us use HOBBYIST or "trade" plants - hybrid, inbred and the like.....

if we are talking F1 "wild" species - I thought the best points were the one that relate photoperiods to specific geographic regions where a given plant is native to. 

Note the latitudes were a plant is found wildly, and considering the natural daylight shifts as the seasons progress - in the Northern Hemisphere about 20min a week light shift climaxing at inverted equinoxes. 

In nature small variants in any given species can evolve drastically in relatively "short" geographic distances due to these kind of variable. Southern New York State gets spring budding 2 -3 weeks before Southern Ontario for example. I can drive there in 6 hours. 

The perfect lighting would be incremental and constantly changing, mimicking the region were it is found naturally.... but as hobbyist not practical- most of the trade plants conmmon in LFS are the result of folks like us who had sucess with them and demand for the breeds spread based on our success as enthusits (and $$ in the till!) - not due to anyone isolating the "perfect" simulation.


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## snafu

MIL007, i can't agree with you more. it makes perfect sense that we would want to have photoperiods that mimick nature (roughly 8-12 hours a day), since plants have evolved over millions of years using the sun as their source of energy with its distinct spectrum, intensity, and photoperiods. actually, that's enough justification for me. i just wanted to understand whether comments i often hear stated as fact were based on research, anedotal evidence, hearsay, or simply incorrect. there are rarely any hard and fast rules when dealing with living things, since living things are extremely adaptable. 
-snafu


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