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Cultivated Reef

Changing PC Bulbs


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I have had a 2x36 hood on my 10 gallon nano for about 9 months. I just ordered replacement bulbs (an actinic and a 10000K) and was wondering when they get here, which should I change first and how long should I wait before changing the second one? Thanks.

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PC bulbs start to "fall off" in output after about 4 months (if running for 12 hours per day.) After 6 months, light output is considerably lower, and after 8 months even more so. After 12 months, output from most manufacturers is down 50%.


You will not be able to see this, because (a) it happens so slowly, and (B) because the human eye is so adaptable that what most people perceive as being "half as bright" or twice as bright" is usually somewhere between a 6x and 8x difference. This is common knowledge in photography, and it's why light meters are necessary.


What you will notice is that after 6 months, coral growth slows while green algae growth increases. This is due to light loss disrupting the balance of the tank, just like adding uncured live rock.


Bulbs run hotter, either by overdriving with "hot" ballasts or being in poorly ventilated enclosures will age somewhat faster. I have 2x28 watt in a small, enclosed Eclipse hood, and they are crap after 6 months. I am replacing them every 4 months fro now on.


Unlike VHOs, I see no reason to replace the bulbs at different times. I have never seen any credible info about different color PC bulbs aging at different rates, as long as everything else is equal.

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I realize that they need to be changed frequently, but I have been busy and haven't gotten around to it. The reason I asked was that I have heard changing both bulbs at once would increase the intensity and could burn the corals that are acclimated to the lower intensity, and not because actinics and 10000K age differntly.

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No, you'r not going to burn anything unless you are running major wattage, and probably then only if the old bulbs were seriously old. I changed a tank from 2x32 watt to 175MH and the corals just jumped up to grab all the light they could.


Keep in mind that in the wild the corals deal with dark days or weeks (storms) and then bright days right after it, and they do fine.

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