There is no such thing as a "freshwater stonefish".
What you're referring to is an oyster toadfish. They're closely related to saltwater toadfish, such as the common aquarium fish "Orange Toadfish", but they have a unique adaptation that allows them to live in brackish and freshwater.
Yes, I've heard of them being kept in freshwater for long periods of time (a year or so), but this is very rare and ultimately they need to be in saltwater or nearly saltwater.
I've had my oyster toad "Leon" for about a year and a half, and he's lived in a saltwater aquarium for about a year now. He was 4" when I bought him, and he has grown to 7.75" ever since I started keeping him in a marine system. So far, I haven't heard of a person having greater success with these fish (although I'm sure there's someone out there).
Sent from my iPhone using
MonsterAquariaNetwork app