Welcome to the forum!
I have about 6 tanks and 2 of them are fresh and the rest are salt. I've been keeping fw
fish for over 20yrs and just a year or two ago decided I wanted salt water. LOL
Uf you want a reef tank with corals then the initital outlay $$$ will be, to be honest, very shocking. The lights are obscenely priced usually and the other initital costs can be alot more than FW
However, i have found that after you have all the necessary equipment and get into a groove the costs are not much higher than FW
and the work is almost the same.
Two of the most important pieces of gear you'll need for sw
would be a refractometer to test salinity and a ro
unit to make your own water at home. Cuts the cost down to pennies a gallon.
Hope that helped a bit!