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I know there are specs on when MH and T5s lights need to be change. I've heard that the spectrum shift starts before this. When do you change your bulbs?
As per a reading on reefkeeper magazine, they did a test with 8 14k Phoenix MH's for one year. The conclusion was this. Most bulbs are rated for 6000 hours. If you do the math at 10 hours a day ( I only run mine 6 ) it would take 600 days for the bulb to go. However, we are all interested in a decent return on the lighting it provides. As per the testing the bulb declined 23% in the first 4 months. However, after 6 months up to and including 1 year the decline of the par/bulb was only another 6%. So basically one would suggest that a year is pushing it. However, this is quite the contrary. I personally would not go over a year even though I have the ability to raise and lower my lights. More so for those that have stationary lights at 3 or more inches from the tank that cannot be adjusted. Adding a new bulb may in fact burn corals if care is not taken when replacing a bulb that is that old.
I personally like to change my bulbs every 7 months. I keep one the most recent replacement in the closet just in case the new bulb takes a dive. This way I have the old bulb with a good month left on it to get a replacement.
From IceCap:
T-5 HO lamps are not an unbent PC, they are a new technology. I've tested GE 6.5K lamps for the equivalent of 2 years on a reef tank with improper venting of heat and still found you were getting above 90% after 24 months, being overdriven on our VHO ballasts (a 54-watt lamp runs at 85-watts). On a T-5 HO ballast, 3 - 4 years is doable. Don't run these lamps on a ballast that doesn't shut down power to a dead lamp. Also there are many brands of T-5 HO lamps and we'll have to learn what's good and what's not.
MH is a bit more messy but using electronic ballasts lowers the initial starting voltage and if it's a high frequency ballast, eliminates the constant on/off of the bulb's arc. Those are the main factors of MH bulb life. Heat is not a factor. In fact they thrive in a hot environment (the bulbs, not the ballasts). For most bulbs I would suggest 14 - 18 months. More for an Iwasaki bulb, less for some of the less expensive new imports. MH bulbs do shift color over time and if they're under or over powered will give off different Kelvin characteristics then they were specified to provide.