Q. My question has to do with metal halide lighting and which is better as a saltwater or reef aquarium cover: glass, acrylic or nothing at all? Will the glass or acrylic filter out any of the spectrum? I plan on using a retrofit two-bulb kit on a 90-gallon aquarium, and was wondering what would be the best way to keep from overheating the aquarium, which has a full hood.
A. Both glass and acrylic will alter the spectrum of the light and will also decrease light transmission, but they will do so to varying degrees. Glass will eliminate the majority of ultraviolet (UV) spectrum from the light passing through it. If the glass is thick it may also color the light slightly due to its greenish cast, which will give a yellower or greener appearance to the aquarium. This is most noticeable with thick pieces (e.g., ½ inch or greater).
Acrylic is preferred by many for aquarium construction because it is colorless and does not add this cast to the water. Acrylic will admit much more UV light, but this depends on the type of acrylic you are using. There are several different types available, and they all differ in their characteristics for blocking or transmitting light. Your best bet would be to contact an acrylic manufacturer to find out which types are best for your application.
The biggest drawback with acrylic shields is that the heat from the lighting systems tend to cause them to warp. Acrylic will also yellow over time due to the intense light and heat, altering the spectrum of the light transmitted. This can be avoided to some extent by efficient cooling of the hood via the use of fans to force air through the hood. However, some lamps rely on a specific temperature around the bulb, and excessive cooling can alter their efficiency and color temperature.
If you are asking if the use of a shield in your hood will prevent heat transfer to your saltwater aquarium, the answer is no. The primary reason to use a shield is to protect the lamp from being splashed with water. Cooler aquarium water splashed on a very hot lamp could cause it to shatter, with all the little shards landing in the aquarium. However, I have rarely heard of this happening in hobbyist systems.
Secondly, the lamp could fail catastrophically (i.e., it could implode) without warning, and again the glass shards could fall in the aquarium, or, worse, it might occur when you are working in the aquarium! This is not unheard of, especially with some of the early versions of today’s more popular metal halide lamps.
Finally, glass shields are mandatory when using some of the bare quartz double-ended lamps used in saltwater fish aquariums today, such as the Osram HQI series lamps and those sold for use in the new Giessman and Arcadia fixtures from Europe. Most of the metal halide lamps sold for the aquarium hobby today already come with a glass envelope around the quartz filament and don’t require a glass shield to be used with their fixtures. That is why most of these fixtures use thin sheets of acrylic instead of glass.
The general rule is: if the lamp has no glass envelope, then the fixture must have a glass shield to prevent the transmission of UV light. If it does have an envelope, then acrylic can be used instead provided the fixture is ventilated! If the fixture is not ventilated, you should use glass — acrylic would quickly warp, melt and fail.
The majority of today’s home aquarium metal halide fixtures are ventilated and use plastic shields. Most industrial fixtures are not ventilated and use glass. For safety reasons it is wise to use a shield of some sort and live with the slight reduction in light intensity. To cool the hood I would simply use a small computer fan to blow air into the hood and several ventilation holes to allow the hot air to escape.