The koi may be swimming sideways because it is frightened or suffers from swim bladder loss, which prevents the body from maintaining balance. If you are frightened, you can recover on your own after a period of time. If you suffer from swim bladder loss, it generally will not recover on its own and needs timely treatment.
Solutions to koi swimming sideways:
1. Raising in isolation
When koi swim sideways, they need to be reared in isolation and rested for a period of time.
2. Maintain the water temperature

The daily temperature change should not be too large. The water temperature needs to be maintained at around 25 degrees to help the koi recover to health.
3. Reduce feeding
During this period, it is necessary to reduce feeding, or slowly stop feeding as appropriate, to help the koi recover.
4. Drug treatment

If it is caused by swim bladder loss, you need to feed the koi some anti-inflammatory drugs, and the swim bladder loss can be gradually alleviated.
5. Change the water regularly
If the water quality deteriorates, it will also cause swim bladder loss, so the water needs to be changed in time, usually every three days.