A hospital tank used to treat sick that have been removed from your display tank should have the exact water parameters, including salinity, as the display. If you want to treat the fish with hyposalinity, then you add the fish, again, to salinity that is within normal levels, then, gradually, over 48hrs, reduce the salinity to 1.009.
Fish should not be taken from one salinity condition and suddenly placed into another with a lower or higher salinity. This is very stressful. FW dips is the only situation where this is done, and it is only used for a few minitures of treatment.
Are you have a specific problem?