I'll toss in my personal experience and hope it helps. Keep in mind biased as heck...
On the beginnings of saltwater fish keeping, the all inclusive systems are not required but do have some pro's and con's. For a fish only system, as suggested the freshwater setup is pretty close. Carbon filtering can't touch what a good amount of live rock and sand are capable of. When I had mine running, I left a hang on the back filter running without the carbon cartridge and filled it with live rock rubble. Not so much for filtering but breaking the water to battle the film that tends to develop at the water surface. Where one can get away with 3-5 times water turnover per hour in freshwater, I've always aimed at least 10 times water turnover with salt. Sometimes higher depending on what you have. As such it tends to be easier to add a pump into the tank just for current. This is also a great way to tie in a protein skimmer. I've found hang on the back skimmer's don't pull their weight as well, and even the nano-skimmers are flighty in their effectiveness. Simply broken down, the skimmer will pull the wastes left over from food and fish leavings. You can run a system without, but you'll have to do serious water changes frequently. Adding to the current is easy however, as an overflow line out to a sump/skimmer provides space for filtration and the return line back increases flow.
If you are just going fish only, a nano-system is serious overkill. They also have perks and problems. On the beneficial side, they provide a great beginner space for coral keeping. The stock lights are enough for low maintenance, hardy corals. The system is all inclusive, meaning that you can add and take away various complementary methods without having to re-design your system everytime. In my case, I removed the bioballs, added live rock rubble, a nano-skimmer, a ventilation fan and several filter pads by simple "put in, take out" practice.
On the down side, saltwater is much less forgiving. The lights on the systems will raise the water temperature. Stock biocube raised mine 5F, the red sea max 10F. Many of the fish acceptable for the smaller system like cooler water, 70-80F tops. More often than not my air conditioning has to be on lest the system overheat. With this heat, and the lights so close to the water you'll deal with evaporation. Go with a refratometer for your sg, it will be a life-saver. With anything under 50gal, just to be safe you'll want to keep 10 gallons of saltwater in case of emergency. Always keep RO water on hand for the top offs that will be required. Without corals you shouldn't have to worry about the various chemicals requried for health, so you'll save expense like mad.
In short, from my experience, prebuilt nano's are easy to keep, but you will deal with lot's and lot's of mini-crisis. For a first tank go with the standard system that you have to piecemail together. You will save on financial expenditure and pick up knowledge that will assist in larger builds later on. You'll still have the mini-crisis, but the solutions won't be proprietary replacements. Good luck with your tank, and share some pics of the process.
Swim on!
), if they are do a 50% water change and you can start adding livestock. no corals for at least 6 months though. let the tank become stable first.
concept to me.