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Discussion Starter · #1 ·
OK, so I have a pretty good understand of PAR as it relates to low/high light. However, there is one thing that confuses me a bit. When we talk about low/high light, we're normally talking about the distance the light is from the substrate. If you're in low light range, that means you're at low light at the substrate level. However, what confuses me is that you can potentially me at high light at the upper levels of your tank?

For instance, I have a 20H tank with a 24" Finnex Ray 2 sitting about 20 inches above the substrate. According to Finnex's PAR data I'm at the upper end of the low light range or the lower end of the medium light range (assuming 15-30 PAR is low light). However, if I go up 4-5 inches, I'm in the high light range (assuming >50 PAR is high light) with PAR values getting higher and higher as I reach the surface of the water.

To stay in the low light range, do I need to raise my light high enough so that the PAR is relatively even in the upper levels of my tank? Right now, my light is 6 inches above the surface with a PAR of 165. I can raise the light 6 inches and drop the surface PAR to 74, which is still high light but not very high light. The top 4 inches of my tank would be in the high light range, and the rest of my tank would be in the low to medium light range. Am I understanding this correctly? The reason I'm asking this is that I have staghorn or bba (not sure which one) at the top of my tank, but no algae at all in the middle and bottom levels of my tank.
 

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PAR at surface will be higher than that at substrate with any light.
Is PAR value at substrate where most plant's are rooted/begin, that is of importance.
 

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It's a very common problem these days, but it's by no means correct. The light really should be evenly distributed across the top of the tank BEFORE it travels down the water column.
But that's difficult with a Finnex Ray2, because it produces a fairly narrow beam of light that opens-up in width as it travels down.

The only way to overcome this problem is to increase the height of the lamp until the entire top of the tank is (more or less) evenly lit. From there on the light will travel down the water as a result of internal reflection from the glass walls with little reduction in intensity (PAR)
Difficult to guess what that PAR level will be, but substrate and top level will be similar.

See also: http://www.plantedtank.net/forums/showthread.php?t=545113
 

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The first thread on the Algae section has links, the bottom one of tells you the different kinds of algae and how to deal/w them. Complete/w pictures.
What Greystoe talked about, the light reflecting back of the glass intothe tank...
that's at least part of the reason lots of aquascape type tanks are going short now. Those 20H tanks were designed when T8 bulbs were all they had. The shorter tanks have more evenly distributed light with not so much difference between top and bottom.
I have an LED light up on a shelf in a closet and replaced it with two T8 bulbs
so I could get one at the front and one at the back of the tank because the
beams of the LED were so narrow it looked to me like someone was shining flashlights down into the tank. You could actually see the beams and they were narrow enough
so that no light was reflecting from the glass back into the tank. Made more shadows
than the T8 bulbs did.
But be more concerned about substrate level PAR and work on the algae from the
cause angle. Pictures would help and a picture of the whole thing tank/light etc as
it is now. Not just a picture or close up of the algae it's self.
http://www.plantedtank.net/forums/showthread.php?t=184368
 

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Discussion Starter · #5 ·
It's a very common problem these days, but it's by no means correct. The light really should be evenly distributed across the top of the tank BEFORE it travels down the water column.
But that's difficult with a Finnex Ray2, because it produces a fairly narrow beam of light that opens-up in width as it travels down.

The only way to overcome this problem is to increase the height of the lamp until the entire top of the tank is (more or less) evenly lit. From there on the light will travel down the water as a result of internal reflection from the glass walls with little reduction in intensity (PAR)
Difficult to guess what that PAR level will be, but substrate and top level will be similar.

See also: http://www.plantedtank.net/forums/showthread.php?t=545113
What you said makes lots of sense because I really only notice the algae near the surface directly under the light. Prior to reading your post, I had already decided to raise my fixture 6 inches, and the light distribution along the top definitely looks more even. I think I'm going to play with the height of the fixture until I get the right balance I'm looking for.
 

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When I say X PAR at the substrate means low light, I mean if you have X PAR at the substrate you can treat the tank as a low light tank. I don't mean than X PAR at the individual plant location is low light. Different locations in the tank will have more or less light than X PAR.

Light from any source except the sun loses intensity proportional to square of the distance from the light. This is inevitable because all other sources of light (light from a source that isn't millions of miles away) spreads out as you move away from the source. When we raise a light fixture higher above our tank we are reducing the light intensity proportional to about the square of the distance from where we measure the light. (It isn't exactly the square of the distance because large sources of light, like tubular fluorescents or panels of LEDs, expose more of their surface as a source of light as you move farther from them, reducing the loss of intensity.)

Reflection off the glass tank walls, actually off the glass to air interface, not the water to glass interface, increases the light intensity near the walls, but has little effect near the center of the tank.

One of the biggest benefits from raising the light higher above the tank is that it reduces the ratio of light intensity at the water surface to light intensity at the substrate, making the intensity more uniform in the tank. This can have a big effect on how much algae problems you have to deal with.
 

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Discussion Starter · #7 ·
When I say X PAR at the substrate means low light, I mean if you have X PAR at the substrate you can treat the tank as a low light tank. I don't mean than X PAR at the individual plant location is low light. Different locations in the tank will have more or less light than X PAR.

Light from any source except the sun loses intensity proportional to square of the distance from the light. This is inevitable because all other sources of light (light from a source that isn't millions of miles away) spreads out as you move away from the source. When we raise a light fixture higher above our tank we are reducing the light intensity proportional to about the square of the distance from where we measure the light. (It isn't exactly the square of the distance because large sources of light, like tubular fluorescents or panels of LEDs, expose more of their surface as a source of light as you move farther from them, reducing the loss of intensity.)

Reflection off the glass tank walls, actually off the glass to air interface, not the water to glass interface, increases the light intensity near the walls, but has little effect near the center of the tank.
Thanks for the clarification Hoppy. After looking at the PAR charts for my fixture, I appears that I really did need to raise the fixture to be in the low range. I'll see how that works out for me.
 

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Discussion Starter · #8 ·
The first thread on the Algae section has links, the bottom one of tells you the different kinds of algae and how to deal/w them. Complete/w pictures.
What Greystoe talked about, the light reflecting back of the glass intothe tank...
that's at least part of the reason lots of aquascape type tanks are going short now. Those 20H tanks were designed when T8 bulbs were all they had. The shorter tanks have more evenly distributed light with not so much difference between top and bottom.
I have an LED light up on a shelf in a closet and replaced it with two T8 bulbs
so I could get one at the front and one at the back of the tank because the
beams of the LED were so narrow it looked to me like someone was shining flashlights down into the tank. You could actually see the beams and they were narrow enough
so that no light was reflecting from the glass back into the tank. Made more shadows
than the T8 bulbs did.
But be more concerned about substrate level PAR and work on the algae from the
cause angle. Pictures would help and a picture of the whole thing tank/light etc as
it is now. Not just a picture or close up of the algae it's self.
I'm sure I have either fuzz algae or staghorn algae on my plants that are near the surface of the water directly under the light. Both types of algae apparently result from a nutrient imbalance or low CO2 according to the link you posted. There are plenty of nutrients available (dosing EI), so the issue is probably low CO2 or inconsistent CO2. The CO2 issue won't be an issue if I just raise the fixture. I've already determined that I need to ditch the Hagen cannister I've been using (too small) and go with two 2L bottles instead. I'll have to take care of that once I'm done setting up my new dirted 10g.
 

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One of the biggest benefits from raising the light higher above the tank is that it reduces the ratio of light intensity at the water surface to light intensity at the substrate, making the intensity more uniform in the tank. This can have a big effect on how much algae problems you have to deal with.
This is very helpful. I went from being very successful at growing plants in a 20 long, to upgrading to a 29 and experiencing challenges with algae, and struggling to get stem plants to grow well. Some plants have done poorly or died back. Diatoms have been a huge problem such as I never had in the short tank.

I have ended up pulling out most of my stem plants. It is rather sad. However, all this time I've been searching for answers as to why my two setups were so different. Maybe this helps to explain it.

I do not have C02. My light fixture is a T8 shop light that holds two bulbs. When I run one, I feel like it is not quite enough light. When I run two, I get algae. Maybe what I need to try is raising the light a lot more above the tank, based on the discussion in this thread.
 

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Could try raising the light and or shorten the amount of time light's are on by an hour or two.
You do not mention whether you are feeding the plant's ?
With more light comes more demand from plant's for CO2, fertilizer's.
While plant's can adapt to low CO2,they still need food to grow.
I am not one of those who believe fish food,fish waste, can sustain good growth so I add macro-micro nutrient's in small amount's once a week or two in my own non CO2 enriched tanks.
I know CO2 is what it is in my tank's ,that which is produced naturally by bacterial processes in substrate ,and that produced as by product of fish respiration.
I also know that I am adding enough fertilizer's to benefit the plant's judging from the growth I see. (approx 1/3 EI dosing used in high tech tank's)
That leaves only the light for me to adjust and so long as I don't go with too much light for too long.. the plant's grow at slow steady clip which suit's me fine.
growth in my tank's is measured in week's/month's.
 

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Discussion Starter · #11 ·
Hi roadmaster,

I have been using the EI method of dosing. Also, I definitely raised my light significantly. My Ray 2 was 6 inches above the surface, and now it's double that. I've effectively reduced my par at the top of the tank by 55%. There is still plenty of light at the top of the tank, but I'm hoping the PAR reduction will reduce the nutrient/CO2 demands of my rotala and allow it to beat out the algae. Raising the light doesn't seem to have significantly dimmed my tank either, but I'll have to monitor how things go. In the end, I think I'm going to aim for slower growth and use DIY CO2 + Excel to provide carbon and control minor algae issues.
 

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Discussion Starter · #12 ·
This is very helpful. I went from being very successful at growing plants in a 20 long, to upgrading to a 29 and experiencing challenges with algae, and struggling to get stem plants to grow well. Some plants have done poorly or died back. Diatoms have been a huge problem such as I never had in the short tank.

I have ended up pulling out most of my stem plants. It is rather sad. However, all this time I've been searching for answers as to why my two setups were so different. Maybe this helps to explain it.

I do not have C02. My light fixture is a T8 shop light that holds two bulbs. When I run one, I feel like it is not quite enough light. When I run two, I get algae. Maybe what I need to try is raising the light a lot more above the tank, based on the discussion in this thread.
Yeah, this is exactly the problem I've experienced. Growth looks nice and healthy in the bottom half of my tank where the light is less intense, but I've had issues as the plants have gotten closer to the surface.
 
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