Just an idea - would you be able to set up a tank specially for it? Or a couple of them? You could keep one in a 30g tank because they get to about 6".
Maybe you could put in a mature filter or some of the media from your existing tanks into an internal power filter and slowly go brackish or full marine. If you have a hydrometer and raise the sg from fw by 0.002 units per week the filter bacteria should survive. High end brackish is very roughly 1.018, full marine 1.022 - 1.026 in sg.
I've two Celon puffers and the owner of the LFS told me himself they were fresh water fish. He's pretty experienced in most other areas of fishkeeping. I've seen the change in them as I've added marine salt. The care of Celon puffs and GSP's is identical apart from mine get slightly bigger.
Mine were in FW in the shop and got ich as soon as I got them home which I treated with high temp (30C) and aquarium salt at 1tsp per 5 gallons. And a shed load of aeration/water movement. I would have used copper being unaware that copper would probably have killed them because they are scale less fish. I did a 50% change each day for a month and kept them in FW until it went. It did.
I'm now slowly raising the salinity by 0.002 s.g. units each week until I get to 1.018 at least. That slow so I don't kill off the filter bacteria.
One of them also had IP's, being wild caught but I managed to treat that by injecting shrimp with Praziquantel and feeding it to them.
Slight battle, but they're doing fine now
I feed mine on live crabs, crushed crab legs, snails, small clams and earthworms so they get enough variety. Next is to get some freeze dried krill and soak that in vitamin solution before feeding.
The crabs don't last more than a few seconds...
Hope this helps!
BTW they don't tolerate ammonia or nitrite - need mature filters and big water changes like 50% a week.