I hate to be argumentative, but have you actually seen with your own eyes (not by word of mouth) a P. nattereri that is 15" or more in home aquaria? I haven't. Here is a quote from fishbase: "Max. size: 33.3 cm SL". Since it is SL, we can tack another couple inches onto that, bringing the TL to around 38 cm or about 15". This is maximum recorded size in the wild, not captivity. And you can't expect all fish to reach that size either (genetics, disease, and food supply all come into play). As with many larger predatory fish, red bellies seldom reach their maximum size in the wild in captivity. Red bellys over 12" are pretty rare. I have yet to see one hit 15". Theoretically, yes, you MAY get a 15" red belly in a very large tank if you take very good care of it (pristine water quality, abundant food, little stress). But practically speaking, it probably won't happen.