Well, I think I found it (many thanks to canyoncreek who gave me the right hint). The minnow is a Pseudorasbora parva (alias Stone moroko), a cyprinidae native of Asia, introduced in Europe. It reaches a max length of a little more than 4".
According to this
academic article it's really appreciated by LMB.
This is a great news because I have an inexhaustible source of these minnows in a friend's pond far 1 hour from mine.
