THings you need to 'try it': a topoff unit with sensor: Hydor makes a cheap one, and if you don't have kids or pets knocking into the sensor, it works fine. This supplies fresh water from a bucket that will keep your salinity steady despite evaporation. Then a pump. Filtration via an actual filter is ok, but decaying gunk can spike your nitrates, which fish don't like much and corals REALLY can't stand. More permanent tanks use about 1-2 lbs of 'live rock ---conditioned!!!---for every gallon in the system. And use not silicate sand, but aragonite, which dissolves into usable minerals, like calcium. ---The live rock and sand ARE a filter, colonized by bacteria that eat up waste. You do a 10% salt-water changeout every week. This keeps your trace elements on track, again, important particularly with corals. And you maintain crabs and snails that eat up waste too.
As you go up toward 30 and 50 gallons, you do well to have a second tank, a sump, where your pump sits and where you have a skimmer, which is a fractionating amino-acid remover that causes 'sea foam' to collect in a cup which you throw out. The bigger your tank, the better that needs to be. If you have nitrate you can't get rid of, a better skimmer is usually the answer. The sump sits in the stand below your tank, which has a 'downflow box,' an arrangement which routes water down to the sump by gravity, and up (by pump) again, cleaned and nice. This, with your skimmer and live rock, completely replace the filter you might use in a freshwater tank. If you want corals, they're not hard (especially some of them, that grow like weeds) but they do require suitable (not cheap) lighting. And the stony varieties will require some more mineral additives. Grow? Oh, yes.
You will also need a bare glass third tank, a quarantine, in which to house new fish before letting them into your tank: wild-caught fish may come in with passengers, and unfortunately, if they get into your tank with your live rock and sand and multiply, they will kill those fish and your other fish. These 'passengers' are too small to see, but will usually break out within a week of observation, so you can treat that fish, save it, and know that there is no more problem before it goes into your real tank. There is also a method of using 2 extra tanks (tank transfer method or ttm) and not medicating. Never, never, never use a medication on your 'real' tank! It will kill the bacteria in your live rock and sand and cause your tank to crash.
You can see you can edge yourself into some complicated operation. I recommend starting small (But quarantine!) and see how much you like it before going into a major investment.
Understand that you need limestone rock as full of holes as possible for good marine rock, and the bacteria for freshwater are different from marine. Dead coral skeleton also works.
