I have had 3 green spotted puffers for some time - in a heavy brackish tank. They become more aggressive with age - and you should ideally allow 90-110 liters per fish, with plenty of hiding spaces. IME when approaching 10 cm in length, the aggression really starts to show. (Good example - the one in the pic above seems to have bite marks on him). Please bear in mind: these fish are BRACKISH when young, converting to full SALTWATER when fully grown: Please do not get one for a freshwater tank, as it is likely they will last only a few years at most (if you are very luck). GSP specimens have been kept in brackish conditions for approaching 20 years! If you think your year-long-lived GSP was a success, please consider that it is believed that they do not even reach sexual maturity until they are 5 years old! Pet stores and other internet sites will persuade you that these are easily kept in freshwater - and you may be able to do so for a month or two. But please, do the fish a favour and keep it in the conditions conducive for its health. You will be rewarded with an active fish with irridescent colours and a shining white belly.
I have two gsp puffers in my 380 L tank with two figure eights, two archers and a Botia striata. They are in brackish conditions and they all get along great. I feed mine mussels and clams that I break open slightly, frozen shrimp and mealworms. I feed them every day and skip one day/feeding a week to let them clear their digestive tracts. They are both really healthy and don't show signs of terrible aggression. If anything my figure 8's are more aggressive than them, which I have been told that it is supposed to be the opposite.
I currently Have 2 green spotted puffers, (named Puffy and the Barron) who live very happily with several mollies and a bumble bee cat fish. Puffers live well with peaceful brackish water fish that are larger than them. The reason they may pick on one fish and not others is the fish they are picking on could be weaker, smaller or vulnerable. They live in a 110 L brackish water tank, and I would not recommend using fresh water. The salt (which should be marine salt water) helps your fish's gills stay healthy and happy, while increasing their life span. You should use about one tablespoon per 4 liters (if they are currently in a freshwater tank, introduce the salt slowly). When they are very young it is okay to use freshwater because green spotted puffers are born (hatched) in fresh water, and they slowly migrate to brackish water. If you really care about your fish you should use brackish water, I promise you will see difference in their health.
I have had four puffers for 6 months. They develop personalities as they grow. One of them likes eating only ghost shrimp, the other one eats only snails and goldfish, while the other two eat almost anything. Once, they viciously attacked a piece of aluminium foil which I accidentally dropped into their tank! I feed them mealworms, crickets, fish, shrimp, frogs, earthworms, maggots, bloodworms, brine shrimp, algae discs, carnivore pellets and snails (I raise all the live food by myself). I do not keep them in an aquarium, but actually in a tank with sand rising, sort of like a shore, and I have planted some plants on the 'shore'. Sometimes, for entertainment, I put in a frog on the shore and watch them harpoon out of the water like a torpedo, grab the frog and flip back into the water to enjoy the good catch while everyone fights over it...
There has been a lot of discussion on whether a green-spotted puffer is truely aggressive. Some say it is, and some say it isn't. I had a puffer that lived for 3 years. In the tank I also had 2 serpae tetras, 3 neons, 2 black neons, and a dwarf gourami. The puffer was very peaceful towards all the fish. In fact, I had to move the serpae's out because they picked on the puffer. I kept it well fed with brine shrimp or other meaty foods, and it did fine in my community tank. By the way, my puffers name was Doof.
These pages have enough comments to give the reader a basic idea on the topic. Further comments are still very welcome (through the site's contact form) as long as they provide new and/or advanced information not yet discussed in the existing ones.