A group of sharks died at the Pittsburgh Zoo and PPG aquarium this weekend after dangerous ozone was accidentally added to their water.
According to Pittsburgh's Post Gazette, the accident occurred after the Zoo closed on Saturday during a routine water change which replaces 34,000 litres/7,500 gallons of water with purified seawater stored in a holding tank.
Rather than throwing away lots of expensive salt water, the aquarium recycles it by storing the polluted water from the water change in a holding tank and then treating it with ozone, a potent oxidising agent which can oxidise organic wastes and kill pathogens.
Ordinarily, the ozone is added and then left to naturally dissipate for several days before it is returned to the tank.
However, the water returned to the tank on Saturday still contained residual levels of ozone which caused problems for the resident fish, resulting in the deaths of sharks.
Rescue attemptThe Pittsburgh Post says that staff at the aquarium quickly donned wet suits and dove into the tank to try and help the poisoned fishes.
Connie George of the Pittsburgh Zoo told the paper: "They knew the sharks were swimming at the top and they were trying to get them to swim deeper. It would be more stressful to the to take them out.
"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.
"The ozone sensor on that tank seems to be correct, so we don't think it's a mechanical error."
This is not the first time that ozone has caused problems at this Zoo. In 2000 sharks died when a vendor's ozonizer malfunctioned and cyanide was added to an aquarium.
Other reports also state that residual ozone has also caused problems before.