Sounds like mild brackish is as far as I should go.
Puffers are warm-water, tropical fish, so a temperature of around 78 degrees Fahrenheit is best for them. A substrate of crushed coral or aragonite sand is recommended to keep the pH at a steady level of around 8. Puffers have no scales or gill covers and as a result are extremely sensitive to the toxins present in a cycling tank. As is recommended with all fish, the puffer must be placed into a fully cycled tank, with NO ammonia or nitrite, and low nitrate (under 20 ppm of NO3-N). Although they are mostly found in freshwater in the wild, years of keeping this fish in captivity have shown that they are best kept in low-end brackish conditions. This equates to a specific gravity (S.G.) of around 1.005-1.008
. If kept in freshwater, their immune system is apparently compromised, resulting in disease and early death for a fish that can live into its late teens. The longest-lived documented Figure 8 puffer was over 18 years old! Naturally, it was kept in brackish water.
Most brackish puffers are sold in freshwater at the shops, so it is best to cycle (using a method that does not require fish!) your tank at whatever salinity the puffer is kept at in the store you are purchasing it from (bring your own hydrometer/refractometer to check the S.G.).
After the puffer is comfortable in your tank, you can raise the salinity slowly (corresponding to an increase in S.G. of 0.001 - 0.002 per week), to ensure that you don’t disturb the bio-filter in the tank you just cycled. To achieve brackish conditions, it is important to use marine salt, measured properly with either a hydrometer or refractometer. Be sure to find a hydrometer that starts at 1.000 (freshwater) — as many only cover the saltwater range. SeaTest and Instant Ocean are two suggested brands of hydrometers that start at 1.000, but there are many others to choose from. If you are adding the fish from freshwater to your already-established brackish tank, you can drip-acclimate them, by using an air hose with a knot tied in it, so it drips your tank water slowly into a bucket with the water the fish came in, until the salt levels in the bucket match the level inside your tank. I have found that for low-end brackish water fish, the brand of salt isn’t very important; I go with the cheapest, as long as it is made for marine tanks. Heavy filtration is recommended — it is nearly impossible to have too much. 50% weekly water changes are the best way to keep your puffers healthy and happy