A park ranger gave mouth-to-mouth resuscitation to a number of fish after the tank they were in was slashed. Forty-five "much loved" fish died at Castle Park in Colchester, Essex, on Wednesday while their pond was cleaned and they were in a container.
Park staff rushed to save a number of fish, managing to successfully resuscitate some of the larger ones.
An 18-year-old man has been charged with theft and causing unnecessary suffering to a protected animal.
Colchester Borough Council said most of the water in the tank was lost, resulting in the death of "45 fish - including all of the oldest, rarest and most valuable Koi Carp that had lived in the pond for many years".
A spokesman said the three largest koi carp to perish were all around 30 years old and may have been worth up to £200 each.
"It's very upsetting for the staff - those carp were like children to them, they'd been with them for so long".
Anne Feltham, the council's portfolio holder for business and leisure, said staff had worked extremely hard to try to rescue the fish. "One of the staff actually gave mouth to mouth resuscitation on some of the larger fish which was an absolute eye-opener to me - I wouldn't have even known that was possible."
The charged man will appear at Colchester Magistrates' Court on 22 November.
A 16-year-old boy who was also arrested was released without charge.