Actually I have a young green spotted puffer in a 25 gal fresh water tank with a rope fish right now and a few other misc. med sized fish. Thing I have found is, if the tank is already established with the other fish and the puffer you put in is smaller then they are, it seems to gentle them down a bit. this isnt their terf. so I have yet to have any problems with mine. I do have a hollow rock that the rope fish lives in and can go to if it wants. although the puffer has never bothered him or the others anyway. So if he is already doing well in that tank then putting a puffer in may be ok. Make sure they already have him in brackish water though before you put him in. Mine was unfortunately living in freshwater when I got him and now I will have to get another aqarium soon and convert him to brackish myself. Just thought I would share my own exp. so that you know that getting one of those cute little puffers may still be an option.