Ah, I see me and you are in the same boat.
My Green Spotted Puffer is named Rizzle, and honestly, he's the most amazing fish I've ever had the honor of owning.
Rizz was rescued from Walmart about 4-5 months ago, and I would have been totally lost if I hadn't had help from kind people on the internet who walked me through all the care he needs.
First thing you'll want to do is go get supplies.
Green Spotted Puffers need 30gallons of space per fish. For one fish, you'll be okay with the commonly available 29gallon.
You'll also need Marine Salt (not aquarium salt) and a refractometer.
Filtration wise go with an Aquaclear filter for a 70gallon tank (trust me, you'll thank me for this later)
When adding salt, you don't want to up the S.G. more than 0.003 or you'll end up cycling your tank. Do weekly water changes and add just a bit of salt each time.
Food- Ghost Shrimp are great for puffers, so are snails, your puffer will need a constant diet of snails so his teeth don't get too long, the best way to provide them is to go to like, Petco, Petsmart, and look in the tanks with live plants, you'll see lots of tiny baby snails, the stores will usually give them to you free of charge.
Another great food for puffers is fiddler crab. Basically you're looking for meaty hard shelled foods for him.
Puffers are very dirty and have a very high bio-load, if he's in a 10gallon, be prepared for water changes 2-3 times a week until he's in a 30gallon.
It seems like a lot of work, but it's worth it, I promise.
A great resource for Puffers is The Puffer Forum, it's where I learned everything I know, everyone there is very helpful, and they'll be able to give you great advice.
thepufferforum.com
Good Luck
