Different ways to measure a fish
There are several methods used to describe the length of a fish and this can cause confusion. For fish keepers the important length is the standard length, this is the measurement used when working out basic stocking levels for small community fish and when discussing the adult size of a fish. If we used the total length then it would give the impression that veiltail varieties of a species were twice the size of fish of the same species with an ordinary tail which would obviously be very silly.
The total length is generally used by anglers and in the laws used to govern the minimum size of commercial catches
Other lengths such as head size, eye diameter, fin size ect are used by scientists when describing a fish species which has to be done in great depth.