The spotted puffer fish does best in a 20 gallon or larger tank as the single fish. They are a brackish fish so they do need a bit of salt in their water but not nearly as much as a reef aquarium.
Spotted puffers are difficult to keep - the tank should be no smaller than 40 gallons and they need to eat hard foods such as small snails to maintain their health.
The pot must spacious, oxygenated and full of water plants and pebbles where the fish can hide and seek foods unless the fish may feel its unsecured environment.