Looks a very nice tank. I kept a brackish system a smidgeon larger while at university, and it was great being able to see monos and scats really strut their stuff. Besides catfish, scats and monos, I also kept a spotted gar (Lepisosteus sp.) of some type and a youngish specimen of Arothron hispidus, the dog-face puffer. Decoration consisted of *huge* granite boulders we collected from the beach near Aberdeen.
Anyway, GSPs and puffers are a dicey combination. I have heard stories of success, but I've also heard stories with nasty endings. Monos are probably able to avoid trouble in big tanks (I can't imagine a faster aquarium fish) but puffers can be surprisingly devious. I'd certainly try and ensure there was a safe "open" area where the monos could hang out without other fish creeping up on them.
The cats are going to look awesome in there!
Personally, I consider an SG above 1.012 (~50% seawater) to be nice but not essential for most brackish water fish. Provided the pH and hardness remain high, I can't see any advantages to going from 1.012 to 1.024 with GSPs, though that is often advocated. They aren't marine fish in the wild. Besides, even marine fish do well, if not better, at SG 1.018 in captivity.
Re: water changes. Water changes are required proportional to the amount of food, particularly protein, put into the system. A skimmer will remove a lot of the organic waste, diminishing somewhat the demand on water changes (though ideally you'd do both). Furthermore, carefully regulating how much food you put into the system will help, too. There's a good case for feeding adult GSPs only every other day, for example. I'd recommend getting a nitrate test kit, and then using that to find a happy medium between the nitrate concentration and the expense of water changes. Some pufferfish are a bit more sensitive to nitrates than other fishes, but even so, a nitrate level of 50-100 mg/l would seem to me to be not unreasonable. I've never kept GSPs so hesitate to be adamant about that, so some research might be worthwhile. Scats, monos, catfish, etc., couldn't care less about nitrate levels.
Cheers,
Neale
