Dear Marc: What's the difference between freshwater and saltwater fish?
To start, these two groups of fish, freshwater and saltwater, evolved in different environments. From a fish keeper's point of view, saltwater fish, in general, require larger volumes of water to live since they evolved from life in the ocean. The larger aquariums and higher capacity filtration systems of these tanks require greater financial outlay to keep saltwater fish. 

Since freshwater fish have evolved in lakes, and freshwater rivers and ponds, they will thrive in a smaller well-filtered tank, so if space is an issue, then go with the freshwater fish. 

Special Thanks
Marc Morrone, pet expert
Parrots of the World