Additional topics on metal halide
When choosing a metal halide system for aquarium corals and fish, the following should be taken into consideration:
Corals and fish do not like light shock. When they encounter a sudden change of light intensity, invertebrates close up and fish run to hide. To start a light, electronic ballast "shoot up", while magnetic ballast "push up", electricity to the bulb. In other words, with magnetic ballast, bulb is gradually on and brought to its prime over several minutes of time, creating less light shock than electronic startup light.
Magnetic ballast is usually more expensive and requires slightly more power consumption to start up than electronic one, but generally has much longer life of use. Light bulbs with magnetic ballasts also last longer when used in the same environments.
LED lights are being developed with good ideas for a very broad range of uses. However, for coral reef aquarium use or for the purpose to grow corals, it is still immature to say LED lights surpass metal halide or will replace metal halide lights soon.
LED saves energy, while metal halide still is the closest in resembling natural sunlight and penetrates deeper in water.
LED generally emits less heat than metal halide, but LED for coral reef aquarium must be in high-output (minimum 1 watt each). High-output LED can produce enough temperature to burn itself in an enclosed or partially enclosed housing, therefore it also requires fan to cool it down. Fan is almost always the first component that fails even in a good-quality lighting system. While metal halide bulb, is made to resist high temperature, it emits heat but it usually will not burn itself.
Manufacturers' manuals often describe that their LED have 50,000 hours of life of use in a "normal environment". We must understand: Any place near an aquarium, with moisture coming from underneath, it's not a normal environment. As a result, you probably will have to accept some failed bulbs or spend some time dealing with customer service long before that.
Its difficult to replace or change LED bulbs. In most cases, you have to open up the housing (You are mostly likely not allowed to open the fixture yourself as most manufacturer warranties state if you do so you void your warranty), and fuse tiny connectors and take old LED out then fuse new ones precisely back in place - a difficult task by hand without precision tool. With metal halide bulb, you can just screw it in or snap it on without having to open its housing.
To spread out light evenly and to penetrate even a moderate depth in an aquarium, you need many high-output LED, often in multiple tens, even hundred, to achieve the effect close to that of a metal halide. We all know that good quality high-output LED bulb is extremely expensive. Price of LED lighting fixture by Solaris or FPO, the manufacturer who has closed its business not long ago, started from $999.99 to well beyond $4,000.00.
LED is the future, but not now for coral reef aquarium hobby and at least in our opinion, not for aquarium higher than 20". Many people may wonder why Solaris /FPO had to close their business. They could probably find the answer now.