Yeah, lucky I discovered it that morning.
Barbarossa is now in the 20 l tank and his beautiful fins grew back.
BTW after a few days we moved Barbarossa to that other tank, YoYo started to play up and swam in circles like NUTS. So we got him 2 other YoYo's as company, now they are a happy little shoal. He's like a father towards them 2 small ones.