Mmmmm, never heard of success in keeping panther grouper in brackish or fresh. Even the thread you listed doesn’t go on to keep it in anything other than full salt (unless I missed it) .
would think it’s easier moving everything to the lower limits of full salt if you want to keep true marine species but converting that scat will take a while at that size.
I trust your panther isn’t turning up any time soon cos that will need some acclimatising, especially if it’s coming from full marine and your gonna try brackish.
keep us updated.
another (larger) fish you can put in brackish are the shark catfish that are often sold for fresh water. They need salt as they grow too, but you might need to find some bigger ones.
