The reason for changing the bulbs is that they lose their intensity and begin to show more of different color than they did when they were brand new. Think of the bulbs as being a rainbow. All the main colors are represented but the BLUE is more important to ocean life (10,000K and above). Yellow is more important to plant life (65,000K and below). If we have more blue than yellow, the corals and inverts will thrive. If we have more yellow, the algae will thrive and the corals will suffer. Over time the blue lights shift their intensity and more yellow and less blue is given off. Not enough for you to see it but enough that your corals and your aquarium will start to change. Sometimes you will see algae blooms when you never had one before. Thats a possible sign that your lights need changing. You should change your bulbs out once a year regardless if you notice the change or not. Changing bulbs is something that everyone dreads because of the expense. There are MH, PC, and VHO. With bulbs that have extreme intensity such as Metal Halides and HQI's, care should be taken to not change them all out at once. The reason for this is because your corals and inverts have already gotten used to lights that are not as intense. Changing them all at once could bleach them due stress from too much lighting all at once. Suggestion for this is to change one at a time, and every week or two, add another bulb. Continue to space out your bulb changes a week or two from each other till all of them are changed out. If you do decide to change them out all at once, lower the amount of hours you have them on and over a 3 to 4 week span, extend them back to their full original time on. This is so that they get used to the intensity but not the same amount of time. As you gradually climb your hours on up, they will become more comfortable and less stress will be placed on them. The same follows with pc's and VHO's but all you have to do is lower the amount of time they are on instead of changing one by one over several weeks. ALL LIGHTS SHOULD BE CHANGED OUT AT LEAST ONCE PER YEAR. That depends on how much time you leave them on. If you place your lights on for at least 8 hours a day, it is recommended that you change them once a year. Hope this helps......
