Human error might be to blame for the ozone-poisoning deaths Saturday of all 10 of the black-tip reef sharks at the Pittsburgh Zoo and PPG Aquarium.

Zoo officials were investigating the accident and will decide later when the sharks might be replaced, said spokeswoman Connie George. The American Zoo and Aquarium Association, which sets accreditation standards, will be notified, she said.

The deaths occurred after the zoo closed Saturday, during a routine, weekly backwash procedure that puts purified water into the 90,000-gallon open-ocean aquarium, a two-story exhibit.

During the procedure, 9,000 gallons of water are removed from the aquarium and stored in a holding tank. A recovery tank that holds 9,000 gallons of already treated water is then emptied into the aquarium to replace the volume that was removed.

The holding tank water is then pumped into the recovery tank where it is treated with ozone, a purifier much like chlorine. Although it dissipates within minutes, the procedure is to wait three days before doing another water exchange cycle, Ms. George explained.

But the backwash water used on Saturday still contained ozone, which impaired the fishes' ability to breathe. The water was reintroduced at the top of the tank, where the 3-foot-long, 15-pound black-tip reef sharks typically swim.

"We're still investigating why the ozone was still in the water and why it wasn't detected before it was released into the aquarium," Ms. George said. "The ozone sensor on that [recovery] tank seems to be correct, so we don't think it's a mechanical error."

An aquarist who was keeping an eye on the exhibit during the backwash noticed within 10 minutes that something was wrong.

Two black-tip sharks that looked particularly ill were taken to a holding tank where an aquarist tried to help them keep swimming.

Aquarists also dove into the main aquarium to help the other animals. "They knew the sharks were swimming at the top and they were trying to get them to swim deeper," Ms. George said. "It would be more stressful to the [remaining eight sharks] to take them out."

A neutralizing agent had been added to counteract the ozone, so the exhibit water was soon safe.

Despite the efforts, all 10 black-tip reef sharks and a puffer fish died. More than 100 other animals in the exhibit, including groupers, butterfly fish, angelfish and other kinds of sharks, were not affected, Ms. George said.

Since 2000, more than a dozen sharks had died at PPG Aquarium before Saturday's incident. Two were eaten by other sharks, two were killed by bacterial infections and four died while being de-wormed. A tank salt mixure contaminated with cyanide in a vendor error killed two others. Three sharks and two other fish died due to ozone toxicity.