Amazon puffers do in fact spend some time in brackish waters. They spend most of their time and do best in fresh though, so brackish has to be temporary.
Their cousin c. Psicattus is a brackish fish, so it definitely stands to reason there is high brackish tolerance in amazons too.
Awesome add, I figured since there's a good amount of saltwater puffers, somewhere down the line they must've evolved a mechanism to expel salt, and that probably carries over a little to the freshwater ones. A lot can handle salt and that'd probably be much safer than meds.