The same way brackish fish tend to do better in brackish water, freshwater fish tend to do better in freshwater. Only difference is, brackish fish tend to live in freshwater at various points in their lives, but freshwater fish do not live in brackish water.
I wouldn't recommend putting the fish in brackish water, since ctenopomas prefer soft and acidic freshwater. It would be like putting brackish fish in blackwater.
Not to mention, it would be difficult to acclimate this fish to brackish water quickly. Normally I suggest lowering the salinity of the brackish tank to freshwater slowly before adding new fish, or putting the fish in it's own freshwater tank and slowly raising the salinity to that of your brackish tank over the course of a couple of weeks before introduction.