Saltwater aquarium lighting requirements differ depending on the type of organisms kept in your aquarium: Fish, invertebrates, or corals.
Fish-Only or Fish/Invertebrates
Fish-Only (FO) aquariums require the least lighting. Primarily, the lighting for a fish-only aquarium is for your enjoyment of seeing the fish in the aquarium. The fish only need limited lighting during the daytime. It is easy to set a timer on your light fixture to turn it on for a dawn to dusk light in the aquarium. Some light fixtures will even gradually turn on the light and then dim it slowly at nightfall. For aquariums that also include crustacean invertebrates (crabs, hermit crabs, and shrimps) and snails, they don't require much light at all for them to go about their lives, as many of them are nocturnal. Fish-only tank lighting is sufficient for these invertebrates.
Most aquarium kits include the tank, filter, and lights that are sufficient for a fish-only tank. Many of these kits now include lighting that is suitable for reef tanks with soft corals. The top mini/nano aquariums include kits that have adequate lighting for fish-only and other aquarium types.
Fish-Only With Live Rock Aquariums
Live rock is used as decorations in many saltwater aquariums. As its name implies, it is a coral rock that contains living organisms, such as polyps and algae. FOWLR (Fish-Only With Live Rock) aquariums have a higher lighting requirement Than FO tanks because the other tank occupants need an increased amount of light to function. Macroalgae is a good example of something that will need more light to grow properly. Most saltwater aquarists strive to have a good crop of coralline algae growing in their aquariums. The different species of the coralline algae not only add color to the tank, but they make the tank look more like the ocean. Coralline algae growth is almost entirely dependent on the light supplied it for photosynthesis.
While fluorescent lighting will work well for most FOWLR tanks, the lower operating cost of very efficient LED tank lights is beneficial even if the initial cost of the light fixture is higher than fluorescent units. The lower electricity use of LED lights will meet your needs and save money in the process.
Reef tanks with corals, fish, anemones, and other invertebrates have the highest lighting requirements for saltwater aquariums. Different corals have different lighting requirements. Some of the soft corals, such as mushrooms and zoanthids have relatively low light requirements compared to some of the hard (LPS & SPS) corals, which demand a much higher light intensity to grow and reproduce. Special lighting is required, depending on species of corals, to ensure they thrive and grow.
Choosing a Tank
Back when the only viable choice for growing corals was fluorescent lights hanging over a tank, along came the power compact (PC) fluorescent lights, which produced a lot more light in a smaller fixture. Then metal halides came in vogue as "the only light for growing corals." This thought promptly went by the wayside when it was discovered that LEDs (Light Emitting Diodes) fulfilled the lighting requirements (intensity and proper spectrum) of corals and at the same time were much more efficient than fluorescents or metal halides.
No matter which type of tank (FO, FOWLR, or reef) you are putting together, try to plan for the future when purchasing lighting. Purchasing a higher grade of lighting than you need right now may pay off with saved money in the future when you eventually upgrade to a reef tank (and you will)!