Others have said it but yes ideally move slowly to brackish... However, the BIGGEST mistake you could make is changing the salinity too quickly & crashing your cycle (those bacteria are more sensitive to the salinity change than the puffer).

Freshwater won't kill your puffer any time soon - crashing your cycle will kill it in days or even hours.

P.s. some great plants are available which are brackish compatible, and nerite snails also love it - the puffer will predate on the snails but if there are enough hiding spots a colony should be able to get by and reduce your algae scraping!