The photosynthetic organisms in your aquarium are dependent on your lighting system to deliver the light they need to survive and thrive.

Corals have an alga called zooxanthellae within their tissue. When the zooxanthellae photosynthesize, it produces enough sugar to not only feed the zooxanthellae but the coral as well. To do this properly, they require the proper spectrum of light as well as the proper intensity.

Of course, the spectrum and intensity of your light bulb(s) will fade over time. Often this change is subtle enough that the human eye cannot detect a difference. It is quite the contrary for the organisms requiring light to survive.

That is why it is of utmost importance to change out your aquarium light bulbs on a regular basis. This article will cover the best practices of bulb replacement and provide tips for changing out your bulbs to reduce shock to the photosynthetic organisms in your tank.

The following table contains recommended time frames for replacing the most common types of aquarium light bulbs. Please bear in mind when referencing this table that there are a few other factors to consider before replacing your bulbs, some of which we will cover here. However, this table should provide you with a general idea of how often you should change your aquarium light bulbs.

Bulb Types Life of Bulb*
Normal Output Fluorescent 6-12 months
Very High Output (VHO) 6-12 months
Power Compact (PC) 9-12 months
T5 High Output (T5HO) 9-18 months
Metal Halide 9-12 months
LED (non-moonlight) ~5 years (50,000 hours)

* Manufacturer Recommendations

You've no doubt observed the wide variances in bulb life in this table and are wondering where in these time frames your bulbs would fit.

Let's find out.

Variables to consider when replacing your aquarium light bulbs:

  1. Length of time the bulb is run. This is sometimes overlooked by aquarists. If you run your bulbs 6 hours per day, you probably aren't going to need to change your bulb as frequently as someone who runs their bulb for 10 hours per day. 6 hours per day for 30 days = 180 hours. 10 hours per day for 30 days = 300 hours! Now expand this scenario to a full year: the 6 hour per day person will run their bulb for 2,370 hours; the 10 hour per day person 3,650 hours. That is a significant difference.
  2. Type of ballast. Certain ballasts can "overdrive" aquarium light bulbs to achieve a higher PAR. However, this overdriving can shorten the life of the bulb. For example, one T5 bulb manufacturer recommends replacing their bulbs after 9-12 months of usage. Yet when using an IceCap ballast with these same bulbs they specifically recommend changing them at 9 months. Keep in mind there are some tricks of the trade, like using a fan to blow air across the bulbs to keep them cooler and increase their lifespan.
  3. Bulb orientation. This more or less refers to single-ended metal halide bulbs (SE), also known as mogul based bulbs. Inside the glass envelope of a SE bulb there is an arc tube; within the arc tube, you'll find a nipple. The nipple was created by the manufacturer when they filled the tube with gas. The nipple should be pointed upwards when screwed into the socket of a light fixture. Having the orientation any other way will not only shorten the life of the bulb but may also affect the color output.
  4. Purpose of the light. LED lights are most commonly used for nighttime lighting (referred to in the hobby as moon or lunar lights) but have recently gained traction and popularity as being the main light source for an aquarium. Moon/lunar aquarium lights are not a requirement for photosynthetic organisms in your aquarium, so you really don't need to replace them until they've actually burnt out.
  5. Purpose of the light (take 2!). Another common practice, especially among reef aquarium hobbyists, is the use of supplemental lighting, such as actinic lighting. As you've probably guessed, these lights are used to supplement the main light source, which are often metal halides. Supplemental light bulbs may not need to be changed as frequently due to the fact that the corals will receive most of the beneficial light from metal halides. It is still good practice, however, to change these out at (less) regular intervals.

These are just a few examples of why the effective lifespan of an aquarium light bulb can vary.

Every aquarium system is different. Even when the hardware is identical, like an all-in-one plug and play setup, the livestock inside the tank itself will differ from tank to tank. That is why it is important to look at the bigger picture and take your own habits and inhabitants into consideration when setting up a bulb replacement schedule.

Now, once you've determined it is indeed time to change your bulbs, there are a few other tips and considerations we'd like to offer.

  1. We've already discussed how aquarium light bulbs lose intensity over time. Keep in mind that your corals will acclimate themselves to this drop-off in intensity. Thus, you should give your corals time to adjust to the higher intensity of your new bulbs, especially if you've put off changing your old bulbs longer than the aforementioned recommended time frames. Here are some pointers on how to do that:
    1. Shorten the photoperiod of the lights. While not the best option, shortening the amount of time your corals are exposed to the new higher intensity lighting may help them adapt without shock and awe. Just increase the photoperiod slowly back to normal.
    2. Change a couple of bulbs at a time. This method works better in a fluorescent lighting fixture than a metal halide system. If your system has four bulbs front-to-back (two white, two blue), for example, replace two bulbs (one white, one blue) one month and then the following month change out the other two.
    3. Raise the fixture over the tank. As light travels through air, it decreases in intensity. If you (can) raise your lights ~12" higher than normal and then slowly lower it over the course of one month this will help acclimate your corals to the higher intensity of your new bulbs.
    4. The screen method. This is the preferred method of many hobbyists and is pretty easy to pull off. The idea behind the screen method is to block some of the aquarium light by placing a screen underneath the fixture. You can even use the same fiberglass screening you'd use for a window or door in your home, which is very affordable and can be purchased at most local hardware stores. The way this works is to place 3-4 layers of screen over the top of your aquarium underneath the light fixture. Then simply remove a layer per week over the course of one month.
  2. One of the best ways to gauge when you need to replace your aquarium light bulbs is to record when you first began using them. I can't tell you how many times I've forgotten to write down when I changed my bulbs and have had to dig into my records to find the receipt showing when I bought them. Even then, I don't always immediately install them, which results in a bit of a guessing game. Fluorescent bulbs are cool because you can write the date on them with a Sharpie. PC bulbs have a nice plastic base to write on and most T5 and T12 bulbs have a metal rim you can scribble the date on, too.
  3. I can't stress the importance of this enough: RECYCLE YOUR OLD BULBS. Please do not throw them in the trash. Metal halide bulbs contain 5-7 times the amount of mercury of a fluorescent bulb. If you toss these bulbs into the trash, they will end up in landfills and eventually work their way into our groundwater where they can cause major problems. Manufacturers like IceCap work with Marine Depot and local fish clubs to gather used metal halide bulbs for recycling. Most cities also have recycling centers that accept used fluorescent bulbs and may take metal halides as well.

Some of you may be wary of manufacturer recommendations and might perceive them as a way for these companies to get you to buy more product than you actually need to. Can the average aquarium hobbyist find out if these recommendations are accurate?

The answer is yes.

There are two relatively common and affordable tools that hobbyists can you use to check the validity of these recommendations. The first is a LUX meter, aka the less expensive option, and the PAR meter (more expensive, although there are definitely some good ones available at reasonable prices, like Apogee's PAR meters).

According to Anthony Calfo's Book of Coral Propagation Volume 1, a LUX meter will "essentially just measure the intensity of available light." It will not, however, measure whether that light is useful to photosynthetic organisms. For that Mr. Calfo recommends using a PAR meter. PAR is an acronym for Photosynthetically Active Radiation. A PAR meter "accurately measures the light that promotes photosynthesis."

There's one last thing to mention before bringing this article to a close. Whenever you change the Kelvin rating and/or upgrade your lighting system, this too affects the lighting intensity shining into your aquarium. You should acclimate your corals in this situation as well, especially when you go from a higher Kelvin-rated bulb to a lower rated bulb.

Whether you go by manufacturer recommendations or use a meter to determine when your light bulbs need to be replaced, the photosynthetic organisms in your aquarium will definitely appreciate the time and effort you've taken to ensure they bathe in the best possible lighting for their needs.

Happy reefing and don't forget to recycle your bulbs.