I run a very simple 10 gallon nano housing hermit crabs and a pistol shrimp + goby pair.
Filtration is via a "sump section" I made from 1/4" acrylic. Water enters at the top right where I made a notch, and then goes through baffles before being pumped back into the display. I have pictures and a thread on a website dedicated to nano reef tanks.
I do not run a skimmer. I do use purigen. I try to do weekly to bi weekly water changes depending on how things look. I buy distilled water from the store, but if I went ANY bigger than 10 gallons then I would go with a RO/DI unit.
Salt is sort of a pain in many regards. I went with a FOWLR because I planned on converting to a reef tank if I enjoyed the little tank. I am pretty glad I started small because the maintenance is just a pain lol. The tank is super easy to clean thankfully, but I despise aging/heating salt water as trivial as it is! Be aware of that if you have not kept marine tanks before. You will accumulate some junk by the time you have salt, pump, heater, refractometer, bucket, etc.
A far cry from my freshwater process of siphoning water out of the window and then refilling with a hose connected to the bathroom sink via hose pipe adapter.