# low light tank - below plant compensation point?



## gmartins (16 Mar 2011)

Hi everyone. 

I've been reading a lot in this and other foruns for a while now but this is actually my first post.

After a few years away from this hobby, I decided to give it a try and I'm starting a 3 gallon planted tank.

I'm using the dry start methods mostly to build up some biomass before I fill in the tank. This also allows me to take some time to carefully plan things ahead.

I have been reading a lot about lightning but there are many divergent opinions. I am currently running a 9w PL 7200K lamp approximately 8 cm above the tank. Things are growing, albeit slowly, which is fine. I'm not in a rush.

What I want is to run a low maintenance tank with low light + pressurized co2 + EI. However, I'd like to be able to grow some plants other than mosses and ferns. So far I have Eleocharis parvula, Heminathus callitrichoides and Echinodorus tenellus. And I think that will be it.

There is now a view that high light is overrated and that even plants regarded as high light plants can  be grown under lower light conditions provided that all other parameters are ok. my question is - with pressurized co2 and EI is my light (9w) sufficient to grow the above mentioned plants? I know that eleocharis and tenellus are relatively easy but I am this amount of light may be below the compensation point of HC.

Your suggestion/opinions would be most appreciated

Gustavo


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## ceg4048 (16 Mar 2011)

Hi Gustavo,
                  The thing that's even more overrated than high lighting is the assumption that one can precisely determine the actual energy levels simply by watts. PAR is what grows plants, not watts. So in your case it's impossible for anyone to say, without using a PAR meter whether you are below the Light Compensation Point (LCP) of any plant in that tank. The lighting in the tank controls the speed of growth. If you are below the LCP the plant simply wastes away slowly as it cannot generate more food than it consumes. 

So all you have to do is to give good flow, good distribution and good nutrient levels/CO2 and just watch the plants. If they don't waste, away then you know automatically that you are above LCP. If they stay intact and appear not to grow then this is just a speed issue and you'll know that increasing the light will increase their speed of growth.

LCP for most plants is somewhere between 10-20 micromole per square centimeter per second, which is really very low.

People often fail to give their plants sufficient flow/CO2 and nutrients for the given lighting levels and the plants deteriorate. They then automatically assume that it's because they don't have enough light, when in reality they had too much light.

Cheers,


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## gmartins (16 Mar 2011)

Hi,

Thanks for your quick reply. Unfortunately I do not have a PAR meter.

Would it be wrong to assume that if plants are now growing while emersed they will also grow when submersed and hence I am above LCP? This is a rather small tank so the water column should not absorb much of the light, I would assume. 

cheers,

Gustavo


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## ceg4048 (17 Mar 2011)

Hi,
    Yes, if they are growing now without water then the light falloff when you do add water will be minimal, therefore I believe it's a good indication that you are above LCP.

Cheers,


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## GreenNeedle (17 Mar 2011)

I would suggest that WITH CO2 addition 0.5WPG is fine.  You don't say (or I didn't see) how big your tank is.

Without CO2 addition then a little more light is needed.

AC


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## George Farmer (17 Mar 2011)

Welcome to UKAPS, Gustavo! 



			
				ceg4048 said:
			
		

> Hi,
> Yes, if they are growing now without water then the light falloff when you do add water will be minimal, therefore I believe it's a good indication that you are above LCP.
> 
> Cheers,


Agreed.  

Going off topic slightly - One really interesting and surprising characteristic I noticed during my PAR testing is that PAR drops off more per cm in air than water.  I always assumed water would block more light for any given length than air.

Does anyone understand the science behind this?


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## Tom (17 Mar 2011)

George Farmer said:
			
		

> Going off topic slightly - One really interesting and surprising characteristic I noticed during my PAR testing is that PAR drops off more per cm in air than water.  I always assumed water would block more light for any given length than air.
> 
> Does anyone understand the science behind this?



But then if you think about this, the light from the sun gets to earth fine through. If you start diving down through the sea, the deeper you go, the darker it is. You supposedly lose all red light within a couple of meters, whereas blue light penetrates deeper.

Maybe in aquarium-depths it's not relevant?


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## mdhardy01 (17 Mar 2011)

Maybe also the fact that fresh water and salt water are different 
I know with ei we add salts but these get taken up by the plants whereas in the ocean the salinity is much higher therefore a denser liquid for the light to penetrate 
Just my thoughts
Matt


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## bigmatt (17 Mar 2011)

internal reflection back from tank sides acting to partially focus the PAR? M


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## George Farmer (17 Mar 2011)

Tom said:
			
		

> But then if you think about this, the light from the sun gets to earth fine through. If you start diving down through the sea, the deeper you go, the darker it is.?


This is my point mate.

You would assume that you lose more light through water like you suggest with your example.

However, when I have tested PAR, I am losing more light, per unit distance, through air.

So suspending the light source higher above the water makes more difference to PAR at the substrate than the physical depth of the water.

Has anybody else experienced this, or was I making a huge error when taking my readings?!


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## gmartins (17 Mar 2011)

Thanks for your replies,

Glad to know that my assumption was correct   

SuperColey - It's a 3 gallon (12 litre, should be around 9-10 litre real volume). According to the WPG  I would then get 3 WPG but I know this does not really apply to small tanks and lights other than the T8 or T12 (can't remember) for which the calculations were made.

 George - that's rather intringuing. you should try and repeat the measures. I remember reading in APC forum a post by hoppy that PAR readings were similar with or without water, probably because we are talking about relatively short water columns here. 

As a second option - I know PAR measures the amount of light available for photosyntheses, and so maybe this is not as affected by depth (at least at the scale of cm's) as other parts of the light spectrum (e.g. reds)??? 

cheers, G


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## Bobtastic (17 Mar 2011)

It would be interesting to see how many ppl actually know their PAR levels or have a PAR metre. The ones that I have seen on the internet seem extremely expensive... Is there such a difference in manufacturing that general T5 or T8 tubes produce different PAR levels? Do the levels that a tube produces drop through it's life time?


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## GreenNeedle (17 Mar 2011)

3WPG should be fine with CO2.

On the light in water aspect. I would suspect that there may be a point where this light 'increase' over through the air slows, comes to zero and then reverses.

Maybe to do with pressure or any number of variables.

light substrates may reflect light upward.  A bit like you see with the shallow sea shores around the caribbean etc.

AC


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## ceg4048 (18 Mar 2011)

George Farmer said:
			
		

> Tom said:
> 
> 
> 
> ...


Hi George, it could be, but it's also likely that the sensors are reading total energy from reflections as noted by others. The underneath of the surface of the water reflects some light downwards as well as allowing the light to escape upwards. These specular highlights are not present when the water is absent.

Cheers,


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## tyronegenade (10 Jun 2011)

Hi George et al,

In answer to your questions regarding light transmittance through water vs air:

Water absorbs red light more than yellow and blue and a lot more than air, so, light traveling in a straight line through water will diminish in the red and then the blue... If there are dissolved polyphenolics in the water then the blue is also rapidly absorbed. See http://educationally.narod.ru/lighttranswater.jpg. Essentially, after 30 cm 50% of the red light will have been absorbed by the water.

But now things get complicated... Light intensity drops faster in air than in a fishtank because of internal reflection. Ordinarily the light will dissipate according to an inverse square law so for every ft from the source you have 1.4 the lumens you had before. This doesn't apply in the aquarium as the light reflects of the glass meaning that the light diminishes according to an exponential law of attenuation: I = Io x e^{-ax} where I is what you get, Io is what you started with, e is the natural base (2.7182818...), x the distance in metres and a the attenuation coefficient. Because of the internal reflection of aquariums the standard attenuation coefficients calculated from the graph above cannot be used. The value of this aquarium coefficient varies with the purity of the aquarium water and cleanliness of the glass but it generally much bigger than expected. Using data from Tom Barr, I calculate a value of 1.24 for some tanks.

Now to dispel some myths. Watts can be roughly equatable to PAR. On average, you will get 1.24 PAR/W. See http://www.apsa.co.za/board/index.php?topic=4454.0 . We can neatly calculate how much of this light reaches the surface of your tank: http://www.apsa.co.za/board/index.php?topic=4379.0 (the joy of having an engineer about). Where we lack data is in knowing (1) how much light actually enters the water column and (2) how much light attenuation can you expect and how does it vary with water purity and glass cleanliness. So my model at http://www.apsa.co.za/board/index.php?t ... 3#msg41523 needs a lot of fixing.

We can, based on efficiency of your reflector tell you how much light your plants will actually get at a certain depth within a certain margin of error. Sadly, we lack date to determine that margin of error. George, if you have exentsive data on the subject of light transmission through aquarium water I would love to see it.

Kind regards


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## dw1305 (10 Jun 2011)

Hi all,


> light intensity drops faster in air than in a fish tank because of internal reflection. Ordinarily the light will dissipate according to an inverse square law so for every ft from the source you have 1.4 the lumens you had before. This doesn't apply in the aquarium as the light reflects of the glass meaning that the light diminishes according to an exponential law of attenuation:


 A quality response, I thought the answer for George's conundrum was probably internal reflection, but it is nice to have the science confirm it. 

You can actually see the effects of sediment etc in the water column on light attenuation when you go to the beach.  In the Severn Estuary, with it's huge sediment load, the light is gone within a few centimetres of the water surface, but if you go out to the W. Scotland etc., where the rock is  hard, the sediment load low and the water clarity is very good, you get the Red Algae (which have the pigments (Phycoerythrin) to effectively use "blue" light, and low growth rates) growing down to over 100m depth (bottom of the photic zone). As an example on St. Kilda, or Shetland, the Kelp _Laminaria hyperborea_, a brown alga (Phaeophyceae), and its red epiphytes grow down to about 45m <http://www.marlin.ac.uk/speciesfullreview.php?speciesID=3614>.

cheers Darrel


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## gmartins (10 Jun 2011)

Hi,

What I can really say is that my HC is growing with only 11 watt, albeit growth rates are slow. Im expecting 5 months to get a carpet well done. I actually see it pearling now and then, but especially on WC days.

The tank is 30 x 20 x 20 cm and the hood is 4-5cm above and 3-4 cm of substrate. So roughly 30 cm away from light.

According to the above I would only get 13.64 units of PAR which is remarkably low and that would be at the surface.

My HM, however, grows quite fast. I have to trim it every 2-weeks, but it is taller and so gets more light.

I got pressurized CO2 and doing EI.

cheers, 

GM


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## niru (22 Sep 2011)

ceg4048 said:
			
		

> George Farmer said:
> 
> 
> 
> ...



Another thing to note is that water acts as lens. Particularly water bodies with moving surfaces (aquariums, swimming pools, shallow streams, etc). There, caustics are formed due to the small waves which act as temporary miniature lenses, continuously focusing & de-focusing light from above. In swimming pools & streams, this gives rise to those arc-shaped bright rings which look like some surrealistic  hypnotic play of dark & light. Depending on the size of the box, and the speed/wavelength of the surface disturbances, these patterns of "increased" light exist within our tanks as well. 
I am sure that the PAR numbers when water is standing still will be less than when inside a working aquarium with lots of water movement.

-niru


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