Color Temperature for Light Bulbs

Color temperature refers to spectrum or wavelength of the light bulb. You might notice that some light bulbs that you using around your house give off a vague yellow color and other bulbs which are considered bluer bulbs will often give off a more bluish color. This indicates that the bulbs have a different color temperature or spectrum of light.

So why is it important to have the light to have the correct color temperature?

Important because corals have photosynthetic algae in their cells and these algae rely on the correct color temperature in order to grow. Typically, you want to have a lightbulb that shines around the 427 nm range, since most photosynthesis occurs in that area of the blue spectrum.

So when you buy a bulb, it will have a color temperature rating in a unit called Kelvins. This rating is not a direct measurement of the spectrum but, indirectly indicates the color the bulb will shine.

For aquarium bulbs, a typical bulb that a hobbyist might start out with would be a "5500 K" . This is a fairly yellow bulb. As you go up and Kelving numbers, the bulbs become more bluish. So if your bulb you purchase is a higher numbers, such as 6500 K, then it is going to be a little more blue and little less yellow than the 5500K. Remember blue is primary spectrum we want for photosynthesis. So if you want an even bluer bulb than a 6500K (which actually isnt that blue), then you might want to choose either a 10000Kelvin bulb (known as a 10K) or an even bluer 14 K bulb.

You might just ask why we don't just put really blue lights on the tank since that's where photosynthesis occurs?

Well the reason is that you also need some non-blue (white) bulbs to view the corals properly and in their natural setting in your aquarium. You are going to need some "daylight" type bulbs or white light. So we recommend that you actually give your tank a combination of blue light and a white light.

For most of our corals, we would prefer a color temperature of 10K or 14 K. or possibly some 20Ks is keeping SPS. It's perfectly acceptable to use a combination of these bulbs.

You don't have to buy a new ballast when you change light bulb color temperatures. As long as the WATTAGE is the same (175W, 250 W, 400 W, etc) then you can use any bulb you wish that has the same type of "base". The base is the part of the bulb you screw into the light fixture. For most metals halides, your fixture will be a "mogul" base. They have some other variations, but by far the most common is the mogul base for metal halides. We recommend you start out with this.

Having the correct color temperature or spectrum is important in allowing the corals to grow well.Often times people base their lighting choices upon what they see visually. But keep in mind the actual color and intensity being emitted by the bulb may be very different from what your eyes perceive.

You might look at a tank of that has blue bulbs on it who can think that is very dim and not giving out enough light for the corals when in fact, they might have more than enough PAR value for the corals to grow and be healthy.

How Often Do I change bulbs?

In a lightbulb that is older, it may appear to be working just fine, but in fact may have shifted its color temperature and gone from a beneficial blue-color towards a non-beneficial yellowish color or even reddish color. So you want to replace your bulbs on a regular basis, even in their still lighting, because if you don't, you are not offering your corals the optimum light spectrum.

So you can get a color shift in the spectrum away from the blue collar as the bulbs get older. Typically male halides will last about a year and put out the proper spectrum until they significantly color shift the age old is different. But this is a rule of thumb and fluorescent tubes will last about 12-18 months before they significantly color shift and will need to be replaced.

We have found that using older bulbs that might actually encourage the growth of hair algae. It appears that sometimes we can change old bulbs and this

It's possible that the reason you see less hair algae in a system with the proper color temperature is because the corals are photosynthesizing more and using the available nutrients and outcompeting the algae for those nutrients. We don't know why, but in our experience it seems to be the case.

In conclusion, the color temperature of a light bulb is very important. If you choose the cheapest lightbulb available, the spectrum will likely not be optimal for photosynthesis. Corals are animals with simple plants (algae) in their cells (zooxanthellae) and these single celled algae, like other plants, require the correct wavelength in order to grow well. So it's very important that you choose the correct color temperature. You also may want to make sure that you choose the correct intensity (known as Wattage) so be sure to check out her section on light bulb intensity also before you buy an aquarium bulb.