These basics are intended for a marine fish aquarium (and basic Nano Reefs), not an advanced marine reef aquarium. A properly set up and maintained salt water aquarium can be relatively easy to keep, in fact, sometimes easier than some freshwater fish such as discus.
These facts and opinions are based on my experience of keeping and professionally maintaining marine fish (and reef) aquariums for over 27 years with one of the largest aquarium maintenance companies in Los Angeles, California.
For Nano reefs much of this basic information applies. I still recommend a UV sterilizer if it can be fitted to you nano reef (a small internal filter such as a Via Aqua 305 Internal Filter” connected to a compact UV works well)
*Cured live rock is a must.
*A fine #00 sand with a ½” layer of #3 sand on top works best in my opinion for cleaning and de-nitrification.
*Water changes and regular checks of water parameters are a must (do not forget to check alkali reserve, the same as KH in FW). Water parameters change more rapidly in a Nano aquarium.
*Two power compact light, changed every six months and cleaned regularly in between. They should be one daylight and one actinic or two 50/50.
 Filtration; Good filtration is a must for a successful marine aquarium. There are many different filters available too.
Canister filters are good for their capacity, but can become Nitrate factories is not rinsed very regularly. I do not recommend Fluvals due to their poor impeller design. Via Aqua. Eheim, Jebo, and Magnum are the ones I recommend.
Wet/Dry filters are good, but usually are poor mechanical filters. The bio ball media in them also should be rinsed regularly in de-chlorinated water to prevent a buildup of organic material, increasing nitrates.
Sump systems with live rock, plants, and sponge filters work well. The live rock is excellent for aerobic filtration (ammonia and nitrite removal) and anaerobic filtration (nitrate removal). The live plants and green algae are good for nitrogen fixing and phosphate removal. The sponge filter is a simple to clean aerobic bio-filter and mechanical filter.
As for live rock, I strongly recommend using it. Make sure it is cured, many stores sell live rock right after it comes to them, and this is not cured live rock. Live rock arrives to the stores wrapped in newspaper and mostly dead by this time, it takes up to 6 weeks to fully cure live rock. Fully cured live rock has the benefit of containing aerobic and anaerobic bacteria; the later helps convert nitrates to nitrogen which is released harmlessly into the atmosphere. Cured live rock also contains many “creatures”, many of which are both interesting and beneficial. You may also create your own using rock high in calcium carbonate, or even dead coral skeletons by placing them under healthy cured live rock for a couple of months in a healthy aquarium (reef set ups are best for this). It is important to use very porous rock for the proper benefits of live rock
Hang on back filters are very limited, but can be used too, especially if combined with other bio filters. Internal filters are also limited, but once again are good combined with others.
Ecosystem mud filtration is effective for nitrate removal (due to the large colonies of anaerobic bacteria), they are much simpler to use than a protein Skimmer in my opinion (which I believe are over sold for fish aquaria).
Unfortunately there are many stores pushing these systems as the end all of filtration, and they are not. They are a good part of a system, but should not be the only part. Good mechanical, other types of bio filtration, and especially germicidal filtration are also important.
More on Nitrate Removal Filtration;
(A) As mentioned above, Mud filters can be very useful for Nitrate removal; you can make a simple one with a HOB filter or sump aquarium. (With the HOB you must place the media in fine mesh or nylon filter bags so as to not cause damage to the impeller, also keep bags out of heavy flow area of filter).  First use a 2 cm layer of #3 gravel on top.  Then 5 cm of #00 sand.  Then you can make your own anaerobic mulm from top soil; rinse fine top soil in a 10/1 bleach solution, then rinse again until clean (you can add a de-chlorinator to remove bleach). This goes on the bottom of the sump in a layer about 2 cm thick.
(B) Also as mentioned above; A lot of cured live rock is extremely helpful for nitrate removal
(C) Plants or green algae (such as caulerpa algae) in aquarium or refugium.
(D) Pre-Filters such as ATIs “Filter Max” on filter intakes; these are easily rinsed and remove organic matter before it can go thru the nitrogen cycle.
(E) Protein Skimmers
(F) Metal Halide lighting. I know this seems off subject, but I have found that Metal Halide lighting helps with nitrate levels, even when other filters are poor. I do not have scientific proof, but I believe the Redox potential (350 mV) this light helps maintain has a lot to do with this.
Filter redundancy is also important, as filters/pumps can break down. Extra filters also increase bio and mechanical filtration. An economical combination would be a sponge filter, internal filter, and a hang on back (power filter).
For more on filtration, please see this site: “Aquarium Filtration”
 Lighting; A 10,000 K Daylight bulb is a start, better would be a 50/50 daylight/ 420nm actinic bulb, or better yet would be one of each.
 Test Kits; an ammonia, nitrite, nitrate, ph, and KH (alkalinity) test kit are all important.
 Water Chemistry; Start with a good marine salt and mix it to a specific gravity of 1.019- 1.021 for fish.
Add trace elements and aquarium buffer. SeaChem makes an excellent product called Reef Calcium which maintains KH, adds bio available polygluconate complexed calcium. Many products available do not add both calcium and magnesium (such as Kalkwasser), and they are BOTH necessary together for proper chemistry and fish health. It should be noted, that unlike freshwater fish which absorb the water around them, marine fish drink the water constantly, which affects their internal body chemistry to the surrounding water.
Maintain Ammonia and nitrites at 0, pH at 8.2-8.4, kH at 200 ppm, Nitrates below 20 ppm. Change water regularly using a gravel vacuum, especially in areas of waste accumulation, this will help maintain low nitrates.
 UV Sterilization; UV Sterilizers are in my opinion, not essential, but are VERY important. UV Sterilizers help with disease prevention and also help maintain a proper Redox Potential (oxidation properties of water). The Redox Potential is often overlooked by many aquarists both SW and FW. For more information please see my article about “Aquarium UV Sterilization and how it works “
 Proper Fish and feeding; do not over crowd a marine aquarium. The amount of fish depends on the aquarium surface area and the type of fish. My article “Basic Aquarium Principles” addresses this subject. Feed your marine fish according to the type of food they naturally eat in the wild. Aquatic based foods such as HBH Marine Flake or Spirulina 20 Flake are good generic fish foods for Tangs, angels, clown fish, ECT. (Angels also need sponge in their diet).
This is a basic article, for MUCH more information please see this large article with more resources, pictures, and marine information: “AQUARIUM SALTWATER BASICS; Fish and Nano-Reef