Hello everybody,
The riddle has been solved. A comment on Youtube about the possible place of origin finally got me on the right track. It is most likely Barbodes bantolanensis. The sister species B. manguaoensis would also be possible, but does not seem to show quite as much red. Both species are native to Lake Manguao in the Philippines and are critically endangered.
Unfortunately I don't see any chance that they will be imported again.