I received a PM on what is coral bleaching and it is a good question. Simply put it is when a coral expels the symbiotic algae,
Zooxanthellae, that provide nutrition and, at the same time, provide color to the coral. Basically, the nice colorful coral you purchased gradually become white. The downside of this is that, not only is your coral unsightly, it will starve to death over time.
There are people who claim that when they switch to brighter lighting, such as MH, their coral undergo bleaching. This may be true for some lower light corals from deeper depths. I find it somewhat hard to believe for shallow water light loving corals. No amount of lighting supplied by the aquarist is going to outdo the light provided by the sun. Even people using very high light outputs pale in comparison to the illumination the sun provides.
It is possible that if one places unshielded HQI lights on the tank that the UV emitted could cause this bleaching. A simple pane of glass between the HQI and water solves this problem as the glass blocks the short-wave UV. There may also be an adaptive response. Coral that has become accustomed to lower light levels is shocked by a sudden increase in light and ejects the
Zooxanthellae. This doesn’t mean you should avoid adding MH to an established tank. It just means you should acclimate the tank to the new lighting.
Some people say that you should limit the photoperiod. That may help but I think a better solution is to gradually increase the light over time. You can prop up the hood higher above the tank at first then gradually lower it. Egg crate or nylon screening (careful it is not so close to a MH light that it melts) can be placed between the hood and tank and then slowly removed. MH light can not be dimmed but if you have a dimmer on VHO, use it. If some corals seem to be losing color move them to a less exposed position in the tank.
A picture can be worth a thousand words. Take a picture of your tank before you add new lights and one immediately after adding them. I like to use a regular film camera rather than a digital. Film still has better color rendition than the best digital imagery. Use only the tank lighting and no flash. If you think your coral is bleaching under the new light you can compare it to the before and orginal new light photo. Is it the same as the picture looked when the light was added? If it appears pale then bleaching may be occurring. Reduce the light level for a while and see if it improves. Do remember that if you change bulbs to different K values the tank may appear different than it was before. Take this into consideration when switching bulbs.
Darn Newbies, I told you to only use a colorfast bleach! 