And then there was Light!
Now you have a tank under one arm, and a filter and pump under the other, what's left? Well, plenty actually. For now we will concentrate on lighting. This section needs an entire article devoted to simply light itself. What lighting means to different life forms, how the sun and the ocean work together and what that means to your aquarium. For now, we will keep it as simple as possible, and I'll try to touch on the basic ideas.
Right now, short of cutting a hole in your ceiling or putting your tank outside, replicating the sun can be a challenging and expensive task. First things first, the tank you just purchased more than likely came with a standard fluorescent bulb fixture. These are low power, low cost solutions that most people start with while they are deciding on what higher end system to go with. They come in a variety of sizes and spectrums, which is sufficient for most "fish-only" systems. They do degrade over time which in-turn will alter the spectrum they output. So, change the bulbs when recommended to retain the optimal performance.
The next step up would be HO, High Output, and VHO, Very High Output, fluorescent lights. They offer a more powerful solution to the standard fluorescent bulb. These were quite popular for some time, but have since faded out a bit. They do provide a more powerful light, even enough for some reef tanks, however they are a lot more expensive, cost more to operate and generate a good deal of heat. Now this is typical of any of the high-end light fixtures, but for the cost there are better solutions these days.
The next step up is still in the fluorescent family. Power Compacts or PCs are one of the most popular solutions today for the beginning reef keeper. These operate the same as other fluorescent systems, however they have are typically two bulbs joined together, as opposed to one tube. This allows for more lighting to be incorporated due to the fact that they are more compact in design. PCs offer a wide range of wattages and spectrums enabling the tank to harbor most of the creatures available, with the exception of SPS Hard Corals and most Clams. Their operating costs are relatively low, and the life of the bulb is quite good at 12 - 16 months. As with any high wattage light system, heat can be a factor. Most PC units come with built in cooling fans, but if they are housed within a hood an additional cooling fan may be required. If not, the temperature of the tank may need to be regulated with a chiller. Keep this in mind when jumping up to high power lighting. Chillers are very expensive, and will be necessary if you are unable to keep the water temperature at an acceptable level. The one drawback to PCs is the fact that their output intensity is limited to a certain depth of the tank. This means if you have a tall tank, one over 24 inches, you may have trouble keeping photosynthetic animals near the bottom.
Now, we step away from the world of fluorescents and into Metal Halides. These lights are for the reef keeper where light intensity is a must. The light produced by Metal Halides is powerful enough to penetrate any tank, short or tall allowing you to keep even the most light demanding of animals, including SPS Corals and Maxima Clams. However, do to their high cost to purchase and operate, not to mention you will need a chiller more than likely, it can be very expensive to make that jump.
The other option in this range is just as expensive. HQIs or Halogen Quartz Iodide lighting is the latest addition to the lighting game. They are more compact then Metal Halides and produce less heat, however the amount is negligible. They still will probably require a chiller and since the output is narrower than that of Metal Halides, they can actually cause damage to some corals directly underneath them. When using these it is best to actually filter the light through a UV coated glass.
Confused on what to get yet? Don't be. The best route for a beginning saltwater aquarist to take is to start by just keeping fish for now. Since fish do not require light to live, then they can be kept with just the standard fluorescent bulb your aquarium probably came with. If you know for sure that you will be keeping corals and anemones, then go ahead and look into PCs if you have the money. If not, then hold off and just concentrate on getting your tank happy and healthy. Just make sure that your bulbs are rated at 10000k to 20000k. This is the spectrum for saltwater. Anything below that will create an environment more prone to unwanted algae growth. Typically 8000k and below is sold for freshwater planted tanks. The usual setup is to have a 50/50 lighting scheme. That means 50% light is 10000k and 50% light is from an Actinic bulb. One, albeit vague, rule of thumb is to have around 1-2 watts per gallon to support the inhabitants of live rock and promote coralline algae growth, and at least 3-5 watts per gallon to support most corals and invertebrates. However, do not take that to heart. There is much debate on the "watts per gallon" concept, and it is warranted. So, think of it as a basic guide at best.