You can easily take a freshwater tank and convert it to salt. The only real thing is the $, the more you spend up front on better equipment the better. You can easily setup a salt tank with just rock, saltwater and a canister filter but it will require maintenance (water monitoring and changes) alot more frequently. If you set it up with a sump/refugium it will add more water volume which adds stability to the water parameters plus added filtration (live rock, sand, macro algae). Problem is that it all costs money. I had a 55g running a hob skimmer and a canister filter with powerheads to achieve proper flow and ran it this way for a year. Tank was successful but required frequent water changes. I just transferred everything to a reef ready 90g and used the 55g as a sump/fuge and it has already been going better than the original setup. It all boils down to what you want to spend and what your willing to make up for financially with elbow grease.