Some of the most exotic species at the National Sea Life Centre in Birmingham could die if electricity is not restored after a black out.
Fish in the tropical tanks are most at risk
The power cut has left temperatures in the tanks at the centre at critical levels, said manager Ian Crabbe.
Supplies were cut to 8,500 businesses and homes in the city centre at 0330 GMT due to a fire in a sub station.
Only eight properties - including the Sea Life Centre - were still not reconnected by Saturday evening.
"There is a grave risk of animals dying," said Mr Crabbe.
Black tipped shark
He said supplies at the centre had been off since about 0100 GMT.
Emergency generators were being used - but they only circulate water in the tanks and do not have enough power to heat the main container which has a 200,000 gallons (one million litres) capacity.
He said that by the middle of the afternoon the water temperature in the tank was dropping at a rate of one degree an hour.
Electricity company Central Network had said supplies would not be restored until well into the night but that put the fish at great risk, he said.
"If we don't get the power back on soon there is the real risk of mortalities," he said.
He added that species like the giant Hawaiian turtle and the black tipped shark were at risk.
Gas heaters were being brought into the building to try and maintain the ambient temperature, he said.
In addition he said a lorry with a heated water tank had been sent from the centre's biology department in Dorset, but that would not have enough room to house all the fish from the centre.
He added that a member of the centre staff would be staying in there overnight to check on the water temperature and the condition of the fish.
A spokesman for Central Networks has said that engineers were working to reconnect supplies as quickly as possible.