ranchu floating

we keep ranchus at home and they were usually bought from very small. we managed to rear them from the size of a 50 cents coin to palm size.

don't know for what reasons, some of them will become sick and cannot swim properly (they tend to flow with the current and turn upside down). they will then eventually die after many many months.

can ranchu experts here tell me what happened to my fishes? i feed them twice a day.