Changing the light environment of your tank may change coral health.
Many reefers forget when they change their bulbs, just how much their old bulbs have lost compared to new ones. There is also a trend to go to a higher Kelvin temperature bulb for better coloration of our corals, the higher K temperature will mean increased UV output of the bulb. Here I will try to describe a few different methods of acclimating your corals to either lighting upgrades (ie; going from PC to MH or increasing the total light of the tank) or replacing old bulbs with new ones.
1) Raise the bulbs
One of the best methods, although not always practical, is to simply raise the height of the lighting 8-12 inches from the target height. This can usually be accomplished easily with pendant fixtures but can be troublesome with lighting contained within a canopy. With a pendant, simply raise the light the desired amount and lower it 1 inch per week until the target height is reached. With canopies, you can use 1 X 4’s stacked across the top edges of your tank to raise the lighting and remove 1 layer of 1 X 4 per week until the target height is reached.
2) Diffuse the light
Another popular method is to diffuse the new light or dim it. This can be accomplished by placing several layers of plastic window screen between the tank
and the light. This can easily be done by cutting a piece of eggcrate (lighting diffuser) to fit the top of your aquarium, lay several layers (6-8) on top of the eggcrate and remove one layer per week or every few days until there is no screen left between the bulbs and the tank. If your lighting is very close to the surface of the water, you need to be careful, some lighting gets VERY hot and could melt the window screen.
3) Reduce the photoperiod
Another method for acclimation is to simply reduce the photoperiod to a few hours per day and increase over the period of a few weeks until the target photoperiod is reached. There is evidence that shows corals must “ramp” up to bleaching due to increased lighting intensity and you can run your lights for 4 hours, turn them off for one, and then turn them back on for another 4 hours. If changing bulbs and using the same manufacturer, and model this will work fine. If changing K temps, or going with a different manufacturer, I would recommend more caution. UV does not need alot of time to burn the corals. Another photo-acclimation method is to reduce the photoperiod to 4 hours per day, 2 hours in the morning and two hours in the evening (the first two and last two of your target photoperiod), increase lighting by one hour per week until target photoperiod is reached. When using this method I like to increase the AM photoperiod by 1/2 hour on Mondays and the PM photoperiod by 1/2 hour on Thursdays.