I have a 3 1/2 inch GSP which is very similar to the Figure 8 puffer. When I bought him, about a year and a half ago he was barely over an inch long. When he reached about 2 inches, in fresh water, he started acting distressed. Some internet research later and we realized it was time to start adding salt to the tank.
At that point he was in a 20g. We started mixing aquarium salt a cup at a time to the tank. To start we would take about a half gallon of water out of the tank mix the salt and put the water back. We did this once or twice a day for about a week.
When it was time for a water change we did a salinity reading with the hygrometer and matched it. We continued this till the fish was happy again.
We have discovered that he is tolerant of changing levels of salt, and this is consistent of his natural habitat, he will show signs of distress if the salt is too high/low. He has not reached full salt yet but he is close.
When we moved in november he got upgraded to a 37g tank in the new place. We tried to match the salt in both tanks before moving and moved the filters and decorations to cycle the tank. He did great!
Keep in mind with puffers they will eat inverts and snails are a necessary part of their diet to keep their tooth worn down.