A UV filter glass is normally used on a metal halide lighting unit. The reason for this is that it prevents any harmful rays from ‘hurting’ the corals.
However do you really need to use one?
Well for me this depends upon the bulb you are using. A lot of metal halide bulbs nowadays come with a UV protection shield built into them, however there are also some which do not.
So before deciding whether to remove the UV filter glass which is built into the metal halide unit ensure that you check your bulb.
There are a lot of theories in the aquarium world at present in relation to UV and any potential damage that it may cause to corals. There are some aquarists who are not using any UV protection at all and are reporting that they are getting phenomenal colours from their hard corals, however what damage is being caused to the corals which we are not aware of – we will probably never know.
I personally do not use a UV filter glass – I have removed it. This is not to test if I get better colouration or not but purely for the reason that I have UV protection built into the bulb I use. I use BLV bulbs in my aquarium.
I could install a bulb without UV protection into my halide unit but am I causing any damage. I may get better colouration, possibly better growth but am I causing pain in any way, am I damaging the corals in any way – as I do not know then for me I refuse to do it.
Personally I will always continue to use UV protection of some form or other. I am sure that some aquarists are getting better colouration from the corals as they are being exposed to rays which in an aquarium they normally do not receive. Whether they receive these rays in the wild is still open for discussion.
I suppose that there are corals in the wild which are very close to the water surface, in some areas there are even corals which are exposed to the open air when the water recedes. In these case then yes these corals must be exposed to ultraviolet rays of some form or other, but…
Are all the corals in your aquarium from the water surface in the wild?
I bet they are not. If, and this is just an if, you only have corals which are from these areas then potentially you could test not using UV protection on your metal halide lighting.
If all of your corals are not from these locations then I would recommend that you use a UV filter glass of some form or other.