I realize Im unearthing a thread from years ago but, I too have a puffer from drs. F&S that is a mere 3.5" long with near adult coloration. When I received him he was about 1.5"-2" long and over the span of about 7 months has only grown 2". He has been in a 90g tank, 0 ammonia, nitrites, and nitrates. I have an aquaponics bed that helps filter out alot as well as making monthly water changes to freshen things up. For the first 4 months before the aquaponics system, I was doing weekly 35% water changes. The growth rate did not seem to change at all since I have added the aquaponics, so I ruled that out as the cause. But who knows, he's healthy fed daily and always soak his food in vitachem, he loves shrimp, mussels, earthworms, snails and even the occasional pellets left over from my cichlids. Its not a issue for me though him being small, he seems to get along with 10 assorted peacocks and haps I have, there was an occational fin nip here and there that I assumed he was the culprit of in the beginning but, I havent seen any for the past few months. He is a very active little bugger compared to most that I see on youtube. He always seems to be scooting along in the tank blowing the sand looking for snails.