I been keeping GSP for many years,
got 3 in full marine, and x5 in two brackish tanks ( x1 low salinity as it contains many plants and other half way to marine )
From my experiance, and lots of research it dont matter what type of enviroment they are in, providing is does contain salt levels.
Marine salt not aquarium salt !!!
I would advise you to buy a hydrometer, marine salt and do a waterchange quickly and gradualy increase salt levels.
RO water is ideal to use if u can obtain this, makes life easier, no need to add conditioners or mix salt with it and it ready straight away, i usually buy 1 container full salt water and 3 freshwater containers and mix to my required salt levels
Helps combat algea too.
If you increase salt levels to full marine, remember you need more equipment like protein skimmers and RO water is vital !
Also how do you know your fish is ill ?
:dive: