Pufferfish come from Salt, brackish, and even fresh water. As a general rule, most pufferfish species from saltwater grow very large(some even getting football size). With your 125gal tank that shouldn't be a problem as long as you dont overcrowd the tank(remember that the amount of live rock,etc you place in the tank will displace a lot of water, which in turn will affect the amount of livestock you can place in the tank), members of the genus Arothon andDiodon are the large ones most commonly found in the saltwater hobby. You indicated you were interested in "small" puffers which depending on your definition of small would steer you towards the fresh and brackish species. The smallest of the salt species is the genus Canthigaster which are also known as Tobys, which can still get up to 4" or 5" in length. The Saddleback Puffer or Toby is the only species I've seen in the stores and seems hard to come by. 4" or 5" is the max. length the largest of the fresh and brackish species(the Fahaka and figure 8 puffer) get, so you see how defining a "small" puffer is kind of relative.