Could be Bitterling, hard to tell without seeing their fins properly.
https://en.wikipedia.org/wiki/European_bitterling
Uuuurrggghhhhh!Looks like a Bitterling to me too.
They have a strange reproductive routine where the female deposits her eggs into a mussel and the male then fertilizes the water around so the mussel sucks it in.
Amazing little fish.
I've never seen a bivalve quoted on this forum before!