An equipment failure at the Calgary Zoo killed 85 fish in a pool they shared with hippos.

The failure of an ozone sensor has confounded zoo officials after they replaced the pool control system during post-flood repairs in the Savannah building in the Destination Africa area.

The malfunction killed nearly half of the pool’s population of tilapia earlier this week, with about 100 of the fish surviving the ordeal.

“We are conducting an investigation into why the system failure occurred,” the zoo said in a statement posted on its Facebook page. “It may or may not have been preventable but we want to make sure it never happens again.”