Greetings. The short answer is "it doesn't matter". Tetraodon fluviatilis and Tetraodon nigroviridis are both freshwater fish that move in and out of brackish water environments their entire life. They DO NOT get born in freshwater and swim to the sea as they mature. That's a total myth. Whether a small baby puffer or a big adult, these fish are perfectly able to be acclimated to anything with the range 1.000 and 1.010.
It is true that in the long term they do better in brackish water than fresh. But this is not unique to puffers: mollies and bumblebee gobies are similar, despite the fact that both of these groups of fish are commonly found spending their entire life cycles in freshwater habitats. It is probable that brackish water provides a healthier environment *under aquarium conditions*, perhaps by reducing the toxicity of nitrate or stabilising the pH through the relatively high levels of carbonate hardness.
In other words, don't fixate on the salinity. It isn't that important. Slowly raise the salinity a little each time you do a water change. Perhaps for the first month, do each 25% water change by removing freshwater and adding water at SG 1.005. This will, after a few weeks, raise the SG of the tank to 1.005 without stressing the biological filter. Leave it at this level for the next 3-6 months. Then, you can start raising the SG a bit more, perhaps to SG 1.008, and then 1.010 a few months later. This will ensure the filter bacteria settle down nicely. Your fish are at FAR MORE risk from the filter bacteria suddenly dying than they are from "the wrong salinity".
You DO NOT need to keep Tetraodon fluviatilis and Tetraodon nigroviridis in seawater tanks. Some people do, and get good results. Quite possibly it is the use of living rock and protein skimmers that makes the difference here. But in terms of saltiness of the water, SG 1.010 is just fine.
Cheers, Neale
