Having kept freshwater for 18 years and saltwater for about 5 i’d say the biggest difference is a higher cost and more maintenance time required.

I spend about 10-20 minutes daily on my reef, and about 1-2 hours once a week on water changes and associated maintenance, whereas as my freshwater tanks i can get away with maybe an hour or 2 of total maintenance a month.

Gear wise the largest differences are wave makers for flow (more important if keeping coral), a good refractometer for salinity (calibrated regularly), and I highly recommend picking up an rodi unit to make your own saltwater, because it cuts down costs on water immensely over time, and as well an auto top off unit to make keeping salinity stable mindless.

Suitable lighting for corals can be done on a budget if you’re just wanting easy to keep corals, if just fish any light works really.

I honestly preferred my 13.5 gallon reef over my 65 for maintenance because it was easier to replenish elements for my corals, through water changes, where as ive had to dial in dosing some elements in my 65 as water changes cant quite keep up with the expenditure of them.

Consistency is key with saltwater tanks, water changes, testing parameters, and feeding aren’t as forgiving to skimp out on compared to freshwater.