PPM = parts per million (salt molecules to water molecules)
To put it simply, the areas I fish are only considered brackish during the dryest of seasons. i.e. lower water = higher salt concentration. And thus most of the times of the year there's not a high enough salt concentartion compared to water to even be considered brackish. And yet I catch Ladyfish, Tarpon, Redfish, black drum, and Flounder in these areas. Fish have an incredible ability to osmoregulate (balance salts and water concentration with in there bodies), and it works both ways. Guppies can be kept in extremely high sailinities (someone correct me if I am wrong), but something like 300 PPM.
Throw a drum in a tank and watch how it goes. Maybe add a dab of aquarium salt, but you should be set.