I have a BG that is swimming sideways,
Maybe he never went to school and learned the proper way to swim!
I agree with Sunil, no worries if it is just one. Could be old age, an eye infection, a damaged fin, or he got hit hard by a LMB but escaped. Probably dozens of other possible reasons.
Do you have any large predator fish in the pond? Usually a forage fish that is injured or swimming poorly draws a lot of attention from the hungry lions.
