Sadly, Pierre passed away over Christmas. He will be greatly missed.
First off, green spotted puffers are brackish fish. Even though many pet stores sell them as freshwater fish, they are not by any means. When keeping your puffers, you must be sure to have some salt in the water. A hygrometer is a must for these fish. I think you can get one for about $4 from a pet store. Puffers are pretty hearty, and can withstand a wide range of salinity values, and can even survive in fresh water for some time, but in fresh water, they won't thrive and will eventually die. I add "Instant Ocean" for salt, and usually keep the salinity between 1.005 and 1.015. In addition, watch out for "salt creep". As the water evaporates, the salinity of the water will increase, so keep an eye on your new hygrometer, and if the salt content gets too high, add fresh water or water with a much lower salt content.
When setting up the tank, I needed some hearty brackish fish to cycle the tank. I added two black Salifin Mollies for about a month before getting my puffer. Mollies tend to be prolific breeders, and in happy conditions they will breed like crazy. I think after about 2 months there was 1 puffer and about 15 mollies in the tank. This leads me to my next point about puffer aggression, they don't like other tankmates. There are now about 7 mollies in the tank now, with absolutely no sign of the missing fish. Once I put a piece of krill into the tank, and one of the mollies was feeling pretty bold, so he decided he wanted to take a bite. The puffer didn't take kindly to this and promptly removed half of the mollies' head, including the gills and an eye. I watched this happen, and it was very quick and quite vicious. The puffer (from here on in known as 'Pierre') watched the molly for a moment and then SNAP, half the fish's head was gone. I figure the rest of the mollies are there because Pierre got to the tank after them, but their numbers are quickly dwindling.
Puffers are primarily carnivorous fish, so they need some space. Pierre is around 3" long in a 15 gallon tank. I don't think I would go much smaller than this. For filtration, he's got an Aquaclear 150 and a Penguin with a biowheel running on back as well as a heater set to 78 (I run 2 filters on all my tanks so if one craps out, I've got a backup).
Pierre is an incredibly cute fish with a very nasty sharp beak which he uses to crack off bits of coral, tear apart plastic plants, and take chunks out of the hood. The beak is actually fused front teeth which in nature the puffer uses to crack open mollusks and crustaceans. If your puffer doesn't have something hard to eat every once and a while, the teeth will continue to grow, and eventually they will interfere with your fish's ability to eat.
Green spotted puffers have an attractive coloration that attracted me to them originally. Under normal conditions, the back of the puffer is a deep green with black spots all over it, with a white belly. This is normal coloration, but like the chameleon, the puffer can change its colors as he or she sees fit. Sometimes instead of a creamy white, Pierre will have a completely black belly, and the spots and colors on his back will fade to very pale. He will also sometimes have spots on his back and a black belly. If your fish has a black belly all the time, he is most likely sick, but if he changes all the time, he's a healthy little guy.
Pierre spends a lot of time sleeping at the bottom of the tank. That is, until he sees me walk into the room, then he will swim up and down on the front of the tank until I feed him or pay some sort of attention to him. The rest of the time, he swims around his tank exploring and nibbling at stuff, every once and a while ripping off chunks of plastic plants and filter downspouts.
I've never seen my fish puff, and probably never will. Puffers don't puff unless they are under severe duress, so I would highly recommend NOT trying to get them to puff. It would probably be fun, but you might injure your puffer, and even if you don't, your fish is going to be pissed at you for a month.
There seems to be a bit of debate on what to feed puffers. My staples are dried and frozen krill, bloodworms (still in the block), shrimp bits, brine shrimp blocks, and flakes. The flakes are actually for the mollies, but Pierre eats them too (although I can't imagine flakes being a lot of food). Puffers can eat A LOT. Pierre will really eat as much as you I can put into the tank, and will swell up to almost twice his size. He will bite up the krill just to the point where he can swallow, and swallow it down. A krill will be consumed in about 2 or 3 seconds. Right after he eats, his belly will be lumpy and misshapen, but this dissipates after he digests a bit.
A live food that I discovered almost by accident is fiddler crabs. I originally wanted something that would scrounge the detritus on the bottom of the tank and could stand the brackish conditions of the tank. I figured their hard shells would make them immune to attack. Boy, was I ever wrong. I tossed 4 fiddlers in the tank. The first one didn't even hit the bottom of the tank. The second one lived about a minute longer. Within about an hour, all four fiddlers were in Pierre's belly, shell and all. It was pretty vicious to watch, as Pierre ripped through the crabs, biting through the shell like it was a softshell crab. The crabs put up a little bit of a fight, but were no match for Pierre's beak-o-death. I now toss crabs into the tank about once a month just to mix things up.
Everything included here is only from my experience of keeping one of nature's perfect predators. A rock-hard beak which will cut through the shell of just about anything; a fish that puffs up to double its size when attacked; and finally, skin and organs loaded with tetrodotoxin, a potent neurotoxin which kills several sushi-eating Japanese every year.
The New Guys!
We picked up a couple new guys from the local Petco, and they are happily settling in. Of course this will require a larger tank. Bummer.