How Much Light Am I Really Getting? - The Planted Tank Forum
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post #1 of 16 (permalink) Old 06-27-2007, 10:19 AM Thread Starter
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How Much Light Am I Really Getting?

I was wondering, if I have a 20G tall tank with 48 watts of light over it, that equates to 2.4 WPG. If I have 3 inches of substrate doesn't that decrease the water volume of the tank? And wouldn't that increase my WPG? I mean, I am not going to get 19.9 gallons of water in there; so when I calculate WPG, shouldn't I consider the area displaced by the substrate?

Plants are also classified by their light requirements, low, medium low, medium, medium high, high, and very high. I've read that below 2 WPG is low, 2-3 is medium, and above 3 is high.

Am I over thinking this, making something simple more complicated than it needs to be?

Just trying to get it right. Selecting suitable plants for the parameters of my tank.

Feel free to opine. Thanks, Joe

20 gallon tall
48 WATT T5HO
AquaClear 30 Power Filter
4 peppered corys, 12 H. Rasboras, and a pair of cherry barbs
1 java fern
* Will be upgrading substrate and adding DIY CO2
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post #2 of 16 (permalink) Old 06-27-2007, 10:27 AM
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The WPG "rule" isn't very exact to start with, so...yea. The displacement of water by the substrate is pretty negligible IMO.

As far as plant classifications, keep it simple. Low, medium, high.

In larger tanks but not too large (20 gallons - ~90 gallons), ~2 WPG is medium to high lighting, and ~2.5 WPG+ is high lighting. I'd call anything lower than 1.8 WPG low.

In small tanks like a 10 gallon or nano tanks, 3, 4WPG isn't uncommon and high WPGs will be needed for growth due to the surface area, blah blah blah. See? The WPG "rule" is more like...a rough guide. It works well enough that we use it, but there are more accurate methods.

Your 48W of T5HO lighting is gonna give you 2.5WPG. You'll probably need to add DIY CO2 which you've planned already...good.

I'd even try some high light plants with this tank...you never know!
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post #3 of 16 (permalink) Old 06-27-2007, 01:24 PM Thread Starter
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Thanks for the feedback. I was just thinking I don't want to buy glosso for the foreground and have it turn out looking like trees.

I guess I will have to do a little more reading on the subject of lighting as it pertains to plant growth.
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post #4 of 16 (permalink) Old 06-27-2007, 07:02 PM
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I hate the wpg rule, it is a terrible rule, its like the inch per gallon rule, both terrible. If you want to get funky get a light lumen tester to see exactly how much light. 2 wpg MH lights beats 2 wpg of T8 NO light, its not only the size of the tank, its what type of light you are using. T5 is pretty good, PC's arent as good as T5's but do the job, MH's are probably a waste of money, T8's and T12's should work but you would need quite a few tubes.
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post #5 of 16 (permalink) Old 06-27-2007, 07:15 PM
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How is MH a waste of money? On large tanks it may be the only effective option. It also has the cool shimmer effect.

In college....so no aquariums for a while.....
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post #6 of 16 (permalink) Old 06-27-2007, 07:44 PM
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I wonder if there's a way to test actual tight hitting the leaves with a light meter in your camera.


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post #7 of 16 (permalink) Old 06-27-2007, 07:52 PM
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How is MH a waste of money? On large tanks it may be the only effective option. It also has the cool shimmer effect.
Well MH can be a waste of money since T5 can be just as good using a bit less wattage and the initial cost is a bit less. But if you want it for shimmer effect go ahead (its why i got an MH for my saltwater tank instead of T5, shimmer) Mh also creates A LOT of heat so you might need fans or a cooler. With fans you need more water to make up for evaporation (RO water) which can raise the cost a bit and chillers use a lot of electricity and chillers cool your tank but heats the room up.
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post #8 of 16 (permalink) Old 06-27-2007, 08:38 PM
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...I guess I will have to do a little more reading on the subject of lighting as it pertains to plant growth.
Here's a decent article about lighting, but it's a little old. Check out the inverse square law section and how it applies to aquariums with good reflectors. Here's one that goes into lux, lumens and PAR. It's a very good article by Ivo Busko.

Tom Barr mentions using PAR as a more accurate way to measure light in aquariums.

Mo' PAR

There many articles about lighting at The Aquatic Gardener.

Happy reading!

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post #9 of 16 (permalink) Old 06-28-2007, 05:11 AM
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Am I over thinking this, making something simple more complicated than it needs to be?



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I guess I will have to do a little more reading on the subject of lighting as it pertains to plant growth.


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Here's a decent article about lighting, but it's a little old. Check out the inverse square law section and how it applies to aquariums with good reflectors. Here's one that goes into lux, lumens and PAR. It's a very good article by Ivo Busko.

Tom Barr mentions using PAR as a more accurate way to measure light in aquariums.

Mo' PAR

There many articles about lighting at The Aquatic Gardener.

Happy reading!

Ahhh, careful what You wish for.......Probably more than You Ever wanted to know about Lighting an Aquarium!


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post #10 of 16 (permalink) Old 06-28-2007, 01:57 PM
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T5 is pretty good, PC's arent as good as T5's but do the job,
I thought PC's were just T5's bent in a "U".

The WPG rule is pretty lacking, but without any special (expensive) equipment, how else can we gauge our lighting, photons per fortnight?

I think about my lighting situation this way: I have a 33 gal. tank with a 55w PC light from AH Supply = 1.66 WPG. I figure with the quality AH reflector, I may be bumped up to 1.66+"x" WPG. Then I tack on the effects of the hazy pane of glass the light must shoot through drops me down to 1.66 +"x"-"y" WPG. I'd be curious to know the real amount of light getting to my plants, but since all aquarium discussions are in WPG terms anyway, it wouldn't be too useful.
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post #11 of 16 (permalink) Old 06-28-2007, 07:00 PM
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they have the same bulb diameter but everything else is different, different phosphors, operates at different temps etc. So bend a T5, take out all the good phosphors and add lower quality phospures, change the ends a bit, and you might have a PC light.

I think the best way to find out what light you have by not spending much is by the eye and experiance. You find out how much light your tank looks like, then you guess what wpg you have that suits you. Lets say you have 1 wpg, but it looks like its 10 wpg, try a few plants, see what works etc, see what happens without CO2 (may have algae) and then estimate the real wpg.

Watts is how much energy is used, not how much energy is outputted.
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post #12 of 16 (permalink) Old 06-28-2007, 07:58 PM
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Originally Posted by Cheesehead Cory View Post
I thought PC's were just T5's bent in a "U".

The WPG rule is pretty lacking, but without any special (expensive) equipment, how else can we gauge our lighting, photons per fortnight?
Lumens per surface area is more accurate. Don't need any special equipment either...just need the manufacturer to actually list how many lumens their lamps "theoretically" put out.

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Originally Posted by musho3210 View Post
they have the same bulb diameter but everything else is different, different phosphors, operates at different temps etc. So bend a T5, take out all the good phosphors and add lower quality phospures, change the ends a bit, and you might have a PC light.

I think the best way to find out what light you have by not spending much is by the eye and experiance. You find out how much light your tank looks like, then you guess what wpg you have that suits you. Lets say you have 1 wpg, but it looks like its 10 wpg, try a few plants, see what works etc, see what happens without CO2 (may have algae) and then estimate the real wpg.

Watts is how much energy is used, not how much energy is outputted.
By eye? Super bright bulbs may not put out enough PAR/PUR for plant growth, rendering them useless....so in a way, I guess I agree with you, but not entirely. If you have experience with a particular line of bulbs, then you can use your eye.
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post #13 of 16 (permalink) Old 06-28-2007, 08:53 PM
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Wouldn't WPG only really count at the substrate level?

The light has to be stronger near the top of the tank then the bottom.
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post #14 of 16 (permalink) Old 06-29-2007, 06:38 AM
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I think about my lighting situation this way: I have a 33 gal. tank with a 55w PC light from AH Supply = 1.66 WPG. I figure with the quality AH reflector, I may be bumped up to 1.66+"x" WPG. Then I tack on the effects of the hazy pane of glass the light must shoot through drops me down to 1.66 +"x"-"y" WPG. I'd be curious to know the real amount of light getting to my plants, but since all aquarium discussions are in WPG terms anyway, it wouldn't be too useful.
Here's two more things to consider as well.

1) Is your 33g aquarium the one that is called a 33 Long 48x13x12?

If this is your tank, notice that your tank is 48" long and that the 55w PC bulb is ~ 22" long. If this 55w fixture is centered; that means that there are some very dim areas at each end of the tank.

Let's say that the light from the 55w PC is spread fairly even over a 28" area. Let's calculate the watts per gallon in this area. So:
55w / [ 33 gal * (28" / 48") ] = 55w / 19.25 gal = 2.86 wpg under the light and each end of the tank approaches 0 wpg.

The whole tank really doesn't get 1.67 wpg. It has more light near the center and very little light at each end.

2) Again, if this is your tank; it's only 12" tall. The plants get more intense light than those in a taller tank.

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post #15 of 16 (permalink) Old 06-29-2007, 02:16 PM
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...just need the manufacturer to actually list how many lumens their lamps "theoretically" put out.
If I could find such info (a sizeable if), assuming it only takes into account the bulb output, wouldn't I also have to take into account the quality of the reflector and the optical qualities of the glass/plastic(?) I'm shooting through? Still sounds pretty ambiguous to me. So we're back to measuring or putting some small amount of trust in WPG, which seems to be the lingo of the hobby anyway. How many forum participants would know whether you have a low/med/high light tank if you gave them your lumens/sq. inch?

Thanks Left C, I have a 36L X 21 H elongated hex tank. I realize that the ends aren't getting as much light. I put my least demanding plants there (java moss at one end, java fern at the other), to hopefully accommodate that.

I would be surprised if the phosphors in my AH Supply light were in any way inferior. "10 WPG to the eye" sounds pretty painful". LOL
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