Amount.
Lighting a Nano or Micro reef is something of a complex issue. While many people feel that they can keep a Nano with the traditional 3-7 watts per gallon of light, others feel that regardless of the watts per gallon ration 30 watts is still only 30 watts.
 

I know of several people who are successfully keeping nanos with lighting on either side of this argument. The original author of the NANO FAQ claims to have heard of someone successfully keeping acropora species in a 2.5 gallon tank lit with only 18 watts of Power Compact lighting. DC Potts (creator of Project:PICO Reef) maintains that his tridacna clams were not happy until they received approximately 50 watts of lighting (which brought him up to around 17 watts per
gallon on the size tank he was maintaining). These contradictory statements make it difficult to determine just what the lighting requirements of a nano actually are.
 

I've done some experimenting which has led me to the opinion that the traditional wattage per gallon rule is, indeed, insufficient on a nano. The first few nanos I set up used the low-end of the lighting spectrum, usually around 3 watts per gallon. All of these setups used Power Compacts. While the inhabitants (species with low to medium light requirements) did okay, none were really thriving. Growth rates were nonexistant. While I feel that the inhabitants of these tanks were in no danger of death through light starvation, they were nevertheless certainly not as healthy as
they could be.
 

About three months ago I removed the 45 watts of Power Compact lighting I had on a 15 gallon tank and replaced them with a 175 watt 10,000K Metal Halide bulb. From 3 watts per gallon the tank went up to just under 12 watts per gallon. The results were spectacularly positive. I have light loving corals (no SPS, though) that are thriving and growing, as well as low light corals (mushrooms) which have also visibly improved with the additional lighting intensity. I mention the mushrooms specifically because they are known to be low-light animals; for all that, they have
not suffered from the high intensity lighting in the least. None of the animals in the tank were scorched or shocked from the additional lighting; a two week period of gradual acclimation was used to prevent light shock or bleaching.
 

What it comes down to is that while you can have a successful coral tank using the standard 3-7 watts per gallon rule, your tank will be that much healthier if you provide larger quantities of light.
 
 

Flourescents.
Let me say right off that I don't like flourescents, be they NO, HO or VHO. Quite simply, they are the least intense form of lighting on a watt-by-watt basis. At the same time, especially on a small tank, they can be much more costly to operate then any of your other options.
 

If you wish to use flourescents on a nano, you are advised to use VHO bulbs. This will mean the purchase of a good ballast, and your setup cost will probably be as high as if you used Power Compacts or Metal Halides. You could use NO bulbs, but you would need quite a lot of them; an absolute minimum of two or three 15 watt bulbs would be needed on a 10 gallon tank, for example, and four or five would be better.
 

You will need to use a mix of actinic "blue" bulbs and daylight "white" bulbs. It's important to buy bulbs intended for use with reef tanks, as only with these will you be sure you are providing the correct spectrums of light neccessary for the health of your corals. Yes, they cost a few dollars more, but they are a requirement. NO output bulbs need be replaced at least once a year; HO and VHO bulbs should be replaced every six months. Since HO and VHO bulbs are not cheap, and since you'll be using a few of them at the same time, bulb replacement can be costly.
 

The IceCap line of electronic ballasts for flourescent fixtures has a very good reputation.
 
 

Power Compacts.
In my opinion, Power Compacts (PCs) are a big step up from regular flourescents on any tank. On small tanks these bulbs really come into their own, however. PCs are significantly brighter and smaller then normal flourescents of the same wattages. They are available in 5700K "white", 7100K "blue" and "red" (for night-time viewing) spectrums. Common wattages that are useful with a nano tank include the 9 watt, 27 watt and 28 watt bulbs. The 27 watt is a quad bulb (twice the standard PC bulb width), which should be taken into account when designing a canopy with
them in mind.
 

Three small fixtures are commonly available that are perfectly designed for use with nano reefs. The first of these is the MiniMite from Coralife, which retails for around $75. This fixture uses 1 9w white bulb and 1 9w blue bulb, mounted in a plastic enclosure. It's best used for really small tanks (such as a 2.5 gallon). I own one and have been reasonably happy with it, but must say that I've found it to be put together poorly. Mine is held together with a couple of rubber bands.
 

Custom SeaLights offers a fixture with a 27w quad "white" bulb and a 9w "blue" bulb, for double the total wattage of the MiniMite. Surprisingly, it retails for about the same amount ($75) as the MiniMite. I have not used one so I cannot comment on their construction, but I have heard very good things about them.
 

Finally, a number of different outfits offer a 15" fixture containing a 28w "white" and a 28w "blue" bulb, for a total of 56 watts. This size is perfect for a 10 gallon aquarium, and may fit on some 5 gallons as well (depending on specific measurements). It's available as both a retrofit kit (for around $130) and a canopy (for around $170). Many people speak very positively about these, as well.
 

The jury is still out over just how often PC bulbs need to be replaced, although it appears that at most they need to be changed once a year. Some individuals report no loss of light or fall off in spectrum for up to 18 months, however. PC bulb costs are slightly higher then VHO bulb costs, but since you need to replace them far less often they are much more economical.
 

Metal Halides.
I think Metal Halides (MH) have a place in the world of Micro and Nano reefs. I am, however, seemingly alone in this opinion. When I expressed my plan to put a 175w MH fixture on my 15 gallon tank I was met with stunned incredulity. I was told the fixture would generate too much heat for such a small tank (wether I used a ventilation fan or not), and that it would be too much light for ANY coral.
 

Well, guess what? I have less of a temperature problem with my MH then I did with my Power Compacts, and without a ventilation fan. As far as the light scorching my corals, my mushrooms (as mentioned above) are thriving.
 

The heat issue shouldn't be brushed off. MH bulbs do generate a lot of heat in a small area. The fixture on my 15 gallon is open, however -- no canopy. In addition, the bulb is mounted 11" above the surface of the water. The tank's temperature is fine like this, even in the summer (I live in L.A.). If I needed to bring it down a degree or two, I could add small muffin fans (available cheaply at Radio Shack) to the fixture.
 

Scorching corals is also not a laughing matter. However, it is easily avoided. Whenever corals are being moved from one intensity of light to another, they need an acclimation period to adjust. This is easily accomplished by either running the light for only a few hours a day and gradually increasing the photoperiod, or by raising the fixture so the bulb is further away, thus attenuating the light. Slowly it can be lowered to it's regular height. New specimens can be placed in shady
areas of an established tank, then gradually moved out and into the open.
 

I'd say a single 175 watt metal halide is good for a 15 or 20 gallon tank. A 100 or 150 watt metal halide would work for a 10 gallon, and a 70 watt fixture for a 5 gallon. I have not had any direct experience with the new 70 watt MH fixtures and bulbs; anyone who has, please email me. I very much want to hear your experiences.
 

Metal Halides provide the absolute brightest intensity of light available currently for reef tanks. They blow the competition away, hands down. The only people I usually hear make negative comments about them are those who have already spent a large sum of money on VHO systems and their upkeep.
 

The bulb of choice for nano and micro keepers should be either the 10000K, 14000K or 20000K bulbs. These bulbs have a lot of blue in their output, obviating the need for supplemental flourescent actinic lighting. Since there's limited room above a small tank, being able to do away with additional bulbs is very useful indeed.
 

Pendant-style fixtures are a good choice, because they leave the tank top 'open', allowing for significant ventilation and heat exchange. They also can be raised and lowered easily, which is another plus. Expect to spend $200-$300 for a 175 watt pendant, depending on the bulb type you get.
 

A small company called Aquatic Lighting Systems (ALS) offers two useful products. A retrofit MH kit, and a modular pendant kit. The retrofit kit requires you to do some simple wiring, as well as to have a canopy (or build one) to mount the fixture in. It's plus is how cheap it is: $75. It comes with a bulb unsuitable for a reef tank, however, so plan on spending an additional $50 to $100 for a good bulb. The modular pendant kit is a 175w fixture in a pendant like casement. Uniquely, it is mounted onto the side walls of the aquarium directly, using struts. It's part of an expandable system, so if you later move to a larger tank you can re-use the fixture rather then scrapping it. I use one of these myself. They cost $200, but again expect to spend another $50-$100 for a good quality bulb.
 

Metal Halide bulbs are expensive, and need to be changed yearly. The 4300K bulbs are not suitable for use in reef tanks, which is a shame as they cost around $20. The 5500K are acceptable but not great, and sell for around $50. The 10000K, 14000K and 20000K bulbs all cost around $100 each. This sounds like a lot, but you're only replacing one bulb a year (on a nano or micro); this actually makes it cheaper then most VHO systems (again, on a nano or micro).
 
 

Conclusion.
The bottom line with reef aquarium lighting is that there's no one "correct" answer. Many possible ways and combinations exist to light any tank, and your actual light requirements depend a lot on your choice of livestock. No application is perfect for every possible tank. Use what your animals require, your budget can manage, your asthetics live with and your personal taste coincide with.