Setting Up

Green Spotted Puffers need a rather specific tank set up to truly thrive. With the right equipment the task of setting up a wonderful home for your puffer can be easy and even fun.
The first thing needed is the tank itself. Green Spotted Puffers need a minimum of 30gallons per fish. Since 29gallon tanks are more easily accessible, they will work fine for a single fish. If buying a new tank is too expensive there are a few other options. People living in the US can post on craigslist.com and often find very cheap (or even free) used tanks. Other good sources for used tanks are the local newspaper and garage sales.
Filtration is very important for keeping the tank clean. In my experience the best filter for a puffer is one that is made for a tank two times the tank size. I use an AquaClear 70 for my 29gallon tank and it works wonderfully. Aquaclear seems to be the brand of choice for most puffer owners. While the tank is still brackish there is no need for a skimmer, but if you choose to keep your GSP at an s.g over 1.020, then a skimmer will be needed.
Lighting isn't too important for a Brackish tank unless you plan on keeping live plants. Not many plants can survive at the high s.g. needed for Green Spotted Puffers. Puffers are notorious for eating live plants, so if you choose to keep any with your fish be prepared to find "hole punch" like bites in them. Some plants that can survive at a lower s.g. are: java fern, java moss, anubias, and hornwort.
When selecting a heater it's best not to buy a cheap brand. The heater should have a dial on it for manually setting the temperature, and also look sturdy. Plastic heaters are usually better since there's no chance of them shattering like a glass heater could. Also remember to pick up a thermometer. Glass thermometers can break, and some puffers have mistaken them for snails and bitten right through them. I use the kind that stick on the glass and they seem to work fine. The temperature should be around 76-78 degrees Fahrenheit.
Before adding any fish to an aquarium the tank needs to be cycled. This builds up beneficial bacterias to support the fish. Many stores sell fish to be used to cycle an aquarium. Using live fish is outdated and cruel, no fish should have to endure the extreme ammonia spikes the cycling process will create. Instead using a fishless cycling method is recommended. When cycling a tank it's best to start out at the s.g. your puffer will need when he comes home from the fish store. A wonderful article on fishless cycling can be found here: http://www.thepufferforum.com/forum/library/water-filtration/fishless-cycling/
Green Spotted Puffers are a high-end brackish fish, this means that they live in water that is partially saltwater. When creating a brackish tank the only salt that can be used is ocean salt, I use the Instant Ocean brand, this can be purchased at a local fish store. Another necessity is a hydrometer. A hydrometer will measure the salt content (specific gravity, or s.g.) of the water. Before purchasing one make sure it starts out at 1.000 and goes all the way to full marine (at least 1.025).
Most Green Spotted Puffers will be sold as freshwater fish, so it's okay to start them out in a freshwater tank and slowly increase the s.g. over time. Before purchasing a puffer ask a manager what s.g. the puffers are kept at, if possible, test the water yourself. If the puffers are being kept in a brackish tank match the s.g. of the store tank before bringing them home.
Always mix the salt in a bucket before adding it to the tank, salt that has not been dissolved can burn, and potentially kill a fish. When raising the s.g. of the tank only raise it by 1.002 a week, any more than this will kill off any beneficial bacterias and cause your tank to cycle again. The exact amount of salt you need to mix with water will be trial and error until you develop your own system.
Green Spotted Puffers are extremely intelligent inquisitive fish that will need plenty of decor and places to explore. Bored puffers will continually pace the side of the glass, to prevent this offer many caves and tall plants (real or fake) for your puffer to swim around. The exact decor is entirely up to the individual designing the aquarium, but in the case of puffers, the more decor the better. Some people will rearrange the decor every once in a while to keep things interesting for their puffers.
The substrate used can be anything from crushed shell, rocks, or sand. Sand will need to be stirred every week to keep pockets of toxic gas from building up. Most people choose to go with a natural look to their aquarium, but a puffer won't care if the substrate is brown or bright purple, half the fun of owning a fishtank is getting to design it.
A background on the tank will help your new puffer to feel more secure and confidant in his surroundings. Darker backgrounds are usually better. A background can be purchased from a local fish store, or a background can be painted directly onto the back of tank (using nontoxic paint). Some people even choose to create a 3D background using foam and cement to create a unique look (http://www.cichlid-forum.com/articles/diy_aquarium_background.php)