They're flexible in their tolerance but, not geared towards constant freshwater. They're more prone to disease when maintained in constant full FW unless the utmost maintenance is kept on their system. Here's passage from a care sheet:
Brackish Water: Freshwater stonefish or whatever they call them in your neck of the woods come from tropical areas where rivers flow into the ocean (estuaries). This means they live in brackish water. When the tide goes out, they live in freshwater. When the tide comes in, they live in river water mixed with ocean water (brackish water). They are flexible little beasts.