I am interested in setting up a 50 gallon tank for a green spotted puffer. He is currently two inches in length. At the moment he is living in one of my freshwater tanks. I understand that these fish require brackish water and eventually full marine. Because of this I plan to, as my puffer grows, transition the new 50 gal tank from brackish to marine. Given the size of my tank and that I plan to transition to full marine as my puffer grows what tank mates would you suggest?

I have done my own research which points to dalmatian mollies preferring brackish conditions and surviving in saltwater conditions but wonder what else would do well with my puffer.

honestly, my experience with puffers is that they dont play nice with others. i have seen a 1 inch puffer eat the barbs off of a two inch cory cat in a freshwater tank. he didnt bother any of the guppies too much, mostly because he couldnt catch them, so i guess you could try something faster than he is?

Anything in a tank with a GSP will eventually die. They are predators.

I've heard that people have kept them with Guppies for years and then suddenly found the Guppies (or half of them, usually) floating around the tank one day because the GSP decided he didn't want to share their tank anymore.

I keep mine with Blue and Red Legged Hermit Crabs (they are marine, but I have them living in 1.015 and doing fine). You need to have plenty of empty shells though (both for the Hermit Crabs to grow into, and to act as decoys so the GSP doesn't know where the crabs are). I also add four dozen Ghost Shrimp every couple of weeks and they'll usually last for a bit but eventually are all eaten.