The thing with marine aquarium salt is it takes at least 20 minutes to fully dissolve, according to the manufacturers. It's actually a mixture of different mineral salts, predominantly sodium chloride but includes many, many others. Most of them dissolve at once, but some don't, and you'll see them sitting at the bottom of the bucket as little white grains. The hotter the water, the faster sea salt dissolves, but some aquarists consider using water from the hot water tank to be dangerous. Because the hot water sits in a copper tank, it has the potential to absorb copper, which is toxic to many invertebrates and a few fish (notably moray eels).
With most brackish water fish getting an exact specific gravity is unimportant, these fish have evolved to be in an environment with a constantly changing salinity. In fact it is probably healthier for them to be exposed to varying salinities: it surpresses external parasites, can stimulate appetite, and is definitely a trigger for spawning in species that are difficult to breed like monos, gobies, and garpike.
However, varying salinities can be bad for the bacteria in your filter. If the change is too great, the bacteria can die, or at least get stressed, and this is obviously bad for the fish.
I've written a little program that helps convert between weight, salinity, and specific gravity called
Brack Calc . It's for Mac and Windows. You might find it useful. As CFC mentions, boxed sea salt is
hygroscopic, that is, absorbs water from the atmosphere unless kept in a dry, air tight container (an old buiscuit tin with a bag or two of silica gel will do fine). Weighing dry salt should be easily accurate enough for hardy brackish water fish, since your error should only be around 5 to 10%, but I concur with CFC on the value of a hydrometer. A cheap floating one is fine for brackish water aquaria.
Of the brackish water puffers, my favourite is
Arothron hispidus, a large species that makes an excellent community fish. It doesn't nip fins and while territorial towards its own kind ignores everything else too big to be viewed as food. But since it grows to at least 20 cm in captivity (twice that in the wild) it isn't viable for your aquaria. I've no personal experience of either green spotted puffers or figure 8 puffers, but I would mention that the former prefer higher salinities to the latter, and that might be a factor in your choice.
The
Brackish FAQ has a section on puffers skimming over topics like salinity preferances and compatability. There are also a few links to some good, much more detailed sites. Though not often mentioned, several
marine species adapt well to brackish water above an SG of 1.010. Take a look at the Aqualog pufferfish book for more details on these. I've done this with
Arothron hispidus, for example. The one species I'd caution you to avoid is the fugu puffer
Takifugu ocellatus that seems quite common nowadays. It's not yet been kept alive for more than a few months in home aquaria, so despite it's lovely colours is best avoided.
Cheers,
Neale
