Staff at the Sunshine Aquarium, in the Ikebukuro neighbourhood, were treating the fish in the tank for a disease outbreak. Oxygenators had reportedly been switched off to increase the efficacy of the medication, leading to a critical shortage of oxygen that killed over 90% of the stock.
Only 73 fish remained alive in the 12 x 10m Lagoon exhibit, which holds around 240 tonnes of water. The tank is usually home to 50 species of fish.
The exhibit has been cleared and cleaned and plans are under way to gradually restock it.
Source : practicalfishkeeping.co.uk