Yeah I have a hydrometer, and right now really liking the dragon gobies and bumble bee gobies,
I looked into the puffer, and apparently when it reaches maturity it require full salt water, only brackish when juvenile
They can live in high end brackish or even low end brackish with a high PH of 8+. My Ceylon is about 4 years old and is now sitting in water with an SG of 1.017. I had him in 1.0005 with high PH until about a year ago.
They can't live with other fish though, they'll attack everything. Well mine does anyways.
Ontopic: Damsels yes blue damsels, they can live on brackish IMO cause there pretty hardy don't kill me for saying it though if it doesn't work for you cause I've seen it in other peoples tanks