If they are the leopard puffer /green spotted puffer, they will need to eventually be around 1.020 specific gravity (pretty much marine salinity) when they reach adulthood, and brackish in between. As babies they are OK in freshwater, although they can be aggressive during feeding time and are very territorial. I would consider them an expert level fish.
I would love to have one, as they are so cool to watch, but it's recommended to basically be in a 29+ gallon by itself, with maybe a few Bumblebee gobies. They must eat snails and small oysters on a half shell to grind down their beak as it never stops growing. They become predators as adults and feeding them live ghost shrimp is what most people do.
Sent from my SAMSUNG-SM-G900A using
Aquarium Advice mobile app