"The larger the fishtank, the larger a fish will grow" is certainly incorrect in general. The ocean is essentially unbounded as far as most species goes and fish have a characteristic size range. I would think that many large lakes would also amount to "essentially unbounded" as far as the large majority of species goes.

I can't find a reference, but I believe that the original claim is primarily about members of the the carp genus (Cyprinidae).

"The larger the fishtank, the larger a fish will grow" is certainly incorrect in general. The ocean is essentially unbounded as far as most species goes and fish have a characteristic size range. I would think that many large lakes would also amount to "essentially unbounded" as far as the large majority of species goes.