From The Aquarium Wiki
What is it?
L numbers are a solution to a problem. Back in the 1980s many unknown catfish (Loricarins) begin to appear in aquarium shops throughout Europe. These fish were so new that they had not been officially identified and so began to be sold with weird common names that varied from shop to shop with no standard.
So help solve this problem, DATZ (Die Aquarien und Terrarienzeitschrift), the German fish magazine, began to try to sort out this mess by labelling them with numbers starting with the letter 'L'. Some had a letter after the number to indicate colour or regional variants.
It was inevitable that there would be overlap and soon there was fish with both numbers.
As each fish is eventually scientifically named, it was hoped the L numbers would be dropped. But the retail trade has been slow in dropping a previously recognised L number. Perhaps believing it would confused their customers.
The rate at describing these fish continues slowly, far slower than the number of newly discovered species found by professional and amateur fish finders. So the practise of numbering continues to this day.
There is a few books available on this subject to try to sort out any confusion.
- See list of L numbers.