I'd have the same question about L. griseus (grey snapper). I've never seen dogs in fresh, whereas greys get right up into river-water, even lower cataracts. How long they stay healthy there, I do'know.
lots of marine species go to fresh water environments to escape predation. in natural environments they can return to higher salinitys. predators are able to find more prey further in from the coastline because of all the young fish thats why you find them in freshwater environments.
you also will most likely have an extremely hard time replicating the mineral content in aquaria that is found in coastal environments which would be important if youre going to keep them in freshwater.
if you want a dogtooth snapper id just keep a marine environment. there are some snappers that you could more safely attempt freshwater(they will still need special accommodation) but theyd still be better off in brackish. have you considered getting a red devil? mine looked very similar to a dogtooth snapper imo.