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#### fishyfishy101

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I have been doing research for upgrading the light on my 56gal from low light to medium light. The tank dememtions in inches are 30L×18D×24H, but let's exclude 4 inches from the hight because of substrate and say 20. That would mean the tank is 10,800 square inches, so that would equal about 7 square meters. I was thinking of using 6500k cfl bulbs. So I looked up PAR ratings for cfls and found a chart that shows a 23watt cfl at 20in away from the substrate at about 34 PAR. So I am assuming that in this instance each watt is roughly 1.5 PAR. Therefore if I used cfls that are 65watt (250watt equivalent) they would be 97.5 PAR each. Now let's say I use 3 bulbs, 97.5 × 3 = 292.5 PAR, and now divide 292.5 by our 7 square meters to equal about 42 PAR per square meter. How does this look? Is all my math correct? It will not have pressurized co2 (yet) but I will be adding excel, so is it possible that this is too much light? I really don't want to have an algae problem. Thx all in advance!

#### Oso Polar

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No, your math is very wrong: tank floor area is completely irrelevant, only distance from light source matters. If your PAR data for CFLs are correct then you should expect about 200 - 300 PAR at 20 in and this will be extremely high light.

Also, I don't think it'll be a good idea to put powerful CFLs right on top of the tank - they'll produce too much heat and light distribution across the tank will not be very good. It will be much better to hang them several inches above the tank. IMHO two 32W CFLs with good reflectors should be enough.

#### fishyfishy101

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No, your math is very wrong: tank floor area is completely irrelevant, only distance from light source matters. If your PAR data for CFLs are correct then you should expect about 200 - 300 PAR at 20 in and this will be extremely high light.

Also, I don't think it'll be a good idea to put powerful CFLs right on top of the tank - they'll produce too much heat and light distribution across the tank will not be very good. It will be much better to hang them several inches above the tank. IMHO two 32W CFLs with good reflectors should be enough.

oh, wow!?! I tried reading up on par but I guess I still don't understand it. :/ why would the tank floor area be "completely irrelevant" you wouldn't add the same amount of light to a 10gal as a 30gal breeder, even though they are both 12in tall. Right? (I'm still learning, be patient)

#### Oso Polar

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It is irrelevant for your calculation because PAR is measured in amount of micromoles of photons per second and square meter, so area is already a part of calculation. This is also called PPFD - photosynthetic photon flux density. It doesn't matter what is the size of the tank (or there is no tank at all) - right in front of the light source the brightness at the same distance will be the same. It is just in one case the whole tank floor will be lighted with this intensity and in another - just one small corner. :grin2: That's why manufacturers of light fixtures usually provide not only one value of PAR for each depth (right below the center of the lamp) but also values for, say, 3 inches off center, 6 inches off center, 12 inches off center etc, for example: http://i45.tinypic.com/2im9rpy.jpg And that's why it is not possible to provide useful PAR values for a light bulb alone - it is impossible to say how the light from this bulb will be distributed over area without knowing the light fixture (reflector) where it'll be installed: bulb light output can be all focused in one small point or it can be diffused across huge area, PAR will be drastically different.

Now, knowing that, say, right below the light it'll produce 100 PAR but 6 inches off center - 50, and tank is 18 inches wide, I can place two fixtures on top: 3 in from the walls, 12 inches between them. This should produce pretty equal coverage with minimum about 75 near the glass. Of course, this is a very rough estimate and fictional example. :wink2:

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