More than 200 fish have died at the National Marine Aquarium in Plymouth after storms caused a power failure.
Water began draining from the Atlantic Reef tank when the life support and back-up systems were knocked out on Tuesday night.
The loss was discovered on Wednesday when aquarium staff arrived for work and found the tank was nearly empty.
A spokesman said despite staff working "tirelessly" to minimise the impact, most of the fish died.
"There are probably about 30 or so that survived, but even some of them are fairly worrying," aquarium curator Paul Cox said.
Conger eels, turbot, bass, pollock, grey mullet, horse mackerel and wrasse were among the all-native species lost.
Mr Cox said a stone bass named Caesar, who was a favourite with staff, also died.
"It's fair to say it's not been our favourite day," he said.
"Yesterday everyone had a job to do and they did it very professionally, but I don't think it really hit them until they got home.
"Not surprisingly, everyone's a bit down today but we've got to keep our eye on the big picture."
Mr Cox said the Atlantic Reef would remain closed until the stock could be replaced, which was likely to take up to two months.
"It's a slow process to restock, because fish as large as these wouldn't be seen in the wild so they're quite unique," he added.
The aquarium said despite such a tragic incident, it would move forward with its plans to refurbish and re-theme the exhibit.