1/ A suitable tank
Generaly Bigger is better , The more water you have on your system the more stable the water parameters, this is particularly relevant to Fish only systems where stocking levels are likely to be high. Currently there is a trend for mini or nanno reef type aquariums these are typicaly stocked with corals and very few fish or none at all. You may wish to have the aquarium drilled enabling you to install a sump or refugim this will be explained more in the folowing section. You should consider carefully all the equipment you intend on using before purchasing a tank to be sure such devices as protein skimmers can be incorporated.
2/ Filtration
Huge topic! In years gone by undergravel filters were common place in marine tanks. Although the undergravel can still work it does have a number of draw backs so it is no longer a favoured system. Extenal cannister filters are very useful and it is possible to run a basic system on cannisters, They are rather high mainenance in that they need regular cleaning to remove excess organics from the system. Unfortunatly despite all the modern design features incorporated into external cannister filters nowadays they are still a pain to open and clean with any real regularity However they are adaptable in that they can be used for mechanical, biological and chemical filtration. Live rock (explained in more detail elsewhere) can be used in reef systems as the sole source of biological filtration. With sufficient circulation in the tank this porous rock will house nitrifying (and denitrifying) bacteria colonies, combined with an effecient protein skimmer this method is favoured by many aquarist and is refered to as the Berlin method. You may also consider installing a Sump tank below the main aquarium . In this instance water overflows via a hole drilled in the tank down into the sump where it can be filtered in a number of ways before being pumped back to the aquarium. One way is to run the overflowed water first through a Trickle filter this is a very efficient method of biological filtration as the bacteria colonies are in a more oxygen rich environment.This system would be best suited to a fish only tank where it has to deal with large quantites of Ammonia. You may decide to use your sump to house a refugim, an area where Marine algae is grown in an effort to use excessive nutrients.
All of the methods I have mentioned (there are others) work . There will be pros and cons to using each of them. Budget, space and of course the intended live stock will all have a bearing on which you choose !
Protein skimmer
Regarded as an essential peice of kit this is not to be overlooked when planning your budget. Protein skimmers are avalaible in a number of guises. Internal, External,Hang on,Air driven ,Power. The principal is the same however the results are not! although internal airdriven skimmers have been used with good results they do need careful regulating and positioning reguarly in order for them to operate at their optimum.Internal airdriven skimmers are to be regarded as a basic skimmer. External skimmers are better as they are power driven this means they incorporate a water pump with an venturi air injector as a result much more air is being injecetd and as a result more protein is removed. Being external also means water levels are constant and they require less tampering with. You should have an idea of what type of skimmer your going to use before you buy the aquarium!
Lighting
For the fish only tank lighting is pretty simple . Regualr T8 flourescent tubes are sufficient for illuminating the tank. A combination of 1 white 1 blue is popular giving a pleasing effect. For the reef tank there are again a number of options. T8 Florescent tubes unless you plan on using lots of these (5 or 6 least) you should consider using T5 Flourescent tubes these have a much higher light output and 4 T5 tubes would be considered sufficient to grow soft and some hard corals under, if space (and budget ) is an issue T5's are the answer. Metal Halides offer us the highest light output and the best results in a reef aquarium stocked with hard and sps (short poly stony) corals. Draw backs include heat and price.
To be continued!
