Most species sold as FW lionfish (there are a few) will tolerate FW but do better in lightly brackish, it doesn't need to be much, the suggested level of 1 TBS/5 gallons will do for most and the majority of other fish tolerate that level quite well. Be careful what you keep with it, they don't move much but can strike quickly and eat suprisingly large fish. I would suggest that everything in the tank be at least a bit bigger than it is.