I had a rough time when I first started with GSP's. I then started doing research and talking with a brackwater expert. For me, I found that marine conditions gave me much healthier GSP's as compared to fresh or brackish.
I wrote this up years ago and here is some pics of my little buddy. GSP will thrive in saltwater and will pretty much survive in fresh. Brackish conditions are the norm. It has been awhile, but here is some info that might guide you some. As far as tank mates went, mine spent about 6 months with a G-Tile eel. This combo has never been proven good or bad to my knowledge if you keep the tank in brackish conditions. That being said, you would need around a 120g tank by adulthood for "2" fish. If you did want to go the eel route, a brackwater expert and myself agreed a spaghetti eel (Moringua raitaborua) would be the ideal candidate. I wanted to go full marine with other options so I had to remove the eel. As a puffer matures, they will become increasingly territorial which is why most dont have tank mates. Most tank mate stories with GSP's end like this....I woke up this morning and found all my fish's heads except my gsp floating around in the tank. While they do look sweet and innocent, they are some crazy killing machines. I use to put crawfish in the tank with pincers open and all. The GSP would usually hover towards the back end of the of the crawdad and crawdad would have pincers above its head in attack or defensive position. In a split second the puffer would dive bomb and take a leg..then another...and another. Once crippled it would hover then target one side of the crawfish remove the pincer towards the body. Then the other and then the head. Those parts would usually be untouched after that and could be immediately removed. For the puffer, he acted like it was a fun game.
Last note: While it sounds weird, my puffer enjoyed watching cartoons. I could sit in my recliner and turn on cartoons and his face was glued to the tank. If I watched tv for a few hours and not cartoons he would usually just go swim around and colors would turn darker. Flip it back on cartoons and would swim to the front of the tank and would just sit there staring at TV and brighten up. I showed this to a few people so I have witnesses and not insane lol. I let brother in law stay at my house once and recieved a call, "Dude, this fish will not stop staring at me no matter where i go in the room. When I walk up to the tank it looks like its aggressive towards me. I got home about an hour later and sat opposite of what I usually did (brother in law was in my seat) and the GSP sat in the front corner of the side I was on and watched TV. If we switched seats he would move to the corner on my side and stare towards the TV. So they are not dummies. They know who gives them food and who doesn't. I had one friend that would come in and he would hide in his cave because my friend would always tap the glass.
A general guide to GSP's
http://www.fishforums.net/index.php?/to ... fer-guide/
Few pics of my young GSP in full marine conditions and yes, that is a smile you see on the puffer in the last pic
http://www.fishforums.net/index.php?/to ... try1153081
here is guide if you dont want to follow links
1. ONLY 1 per tank or dont be suprised if you end up with missing or damaged fish. Fishmates are hard to come by as they are very agressive, they just wear a sweet face. Fish mates are possible and even multiple GSP's. But, be prepared to offer all tankmates PLENTY of space of their own. In other words are 55+g aquarium.
2. A green spotted puffer (aka GSP) is a brackish fish. They are spawned in freshwater but move into brackwater conditions and eventually even marine conditions
3. territorial, as they grow they will become territorial and they do fight to the death
4. feeding: every other day brine shrimp, bloodworms, freeze dried shrimp, ghost shrimp(can survive in water up to 1.010 in my experience)- snails once a week or every other week to keep teeth trimmed down. There are other foods, this is just what i have used. They need the shells to trim their teeth or they will grow so much that they cant eat at all and die. Dont use flake food as this is not part of their diet and is not suitable. Dont be scared of the big belly they get after feeding it is normal.
5. keeping them occupied - they love to explore their terrain. Stacking rocks very high on the rear of the tank with lots of caves and tunnels usually does the trick.
6. Use marine salt not aquarium salt. Marine salt is a must as aquarium salt is not ment for brackish or marine fish to live in but as more as a medice for freshwater fish.
7. Puffers are very shy during their first few weeks in a new enviroment. Do not pressure them to come out if they dont want to. Eventually they recognize you and show you all the personality you could ever ask for in a fish. Just give them time.
8. Acclimate them to salinity slowly. raising the water .002 every week is how i did mine. He is now in 1.021 full marine conditions. (slow acclimation helps keep bacteria alive as freshwater bacteria differs from brack, brack differs from marine). If you already have a tank cycled at a higher salinity, then you can acclimate much faster using a drip method in just a few hours (nmonks).
9. water temp of 78 while in freshwater-brackwater 80 once in brackwater
10. minimum tank of 20g up to 3" adults will need a 30+g tank (nmonks)
11. Heavy filtration. Puffers are messy eaters and often food settles to the bottom. I have a 20g tank with a 55g filter on it. 25-50% water changes weekly with syphon is a must.
I highly suggest getting to know your little guy (or girl) before adding any fellow tankmates to the mix. It will give you a better idea of what will work.