The L-number system is a semi-scientific classification system of catfish based on photographs of shipments of tropical catfish of the family Loricariidae published by the aquarium magazine DATZ (Die Aquarien und Terrarienzeitschrift).
An L-number is not an official scientific name, but it does allow people to identify various Loricariid catfish by a "common name" before the fish is actually officially described. When a Loricariid receives an official scientific name, the L-number is retired.
In general a specific L-number classification does not guarantee a discrete species—multiple L numbers have been given to different populations of the same species. To add to the confusion, sometimes a single L-number may actually be used for multiple species.