I don't know a whole lot about fresh water, but my understanding is they are a completely different science and require completely different equipment. For instance, I was bought a 55g set up for freshwater for my birthday - I went to the LFS
and fell in love with the saltwater fish! I asked the guy can it be done? Of course it can. The only piece of equipment I could use was the heater. Everything else went on eBay
best advise I can give is do plenty of research ask questions here. You will find there are no hard and fast answers because there are no rules as such. Everyone has different set ups depending on what they are keeping. Do you know what you would like? Fish only? Full reef? These can be completely different set ups. Do you have the tank?
Decide what you would like to keep first. Take it slow, be prepared to be patient and to spend a bit o cash