The Deep aquarium at Sammy's Point in Hull had to close in March and needs cash to carry it through the pandemic.
Looking after the 3,000 animals at the aquarium - including seven species of shark - costs £6,500 a day but, with no customers coming through the door, revenues have nose-dived.
The application for a Government-backed Coronavirus Interruption Loan was revealed in a Hull Council decision record.
It stated: "The UK Government is providing a guarantee to NatWest for 80 per cent of the loan, although the Deep is still responsible for payment of the loan in full."
The loan is repayable over four years with no repayments in the first year. However the interest rate will be on commercial terms for the remaining three years.
It adds: "Consequently the company will need to be able to raise the income to meet repayments through its trading activity.
"Prior to the pandemic the surpluses generated by the company would have been adequate to meet the repayments."
Since opening in 2002, the Deep has proved a huge success: it pulls in around 450,000 visitors every year, is a popular destination for corporate entertaining, and acts as a platform for globally significant research and conservation programmes.
It appealed for donations last month, reporting on April 21 that it had raised over £30,000.
One of its most popular exhibits are the gentoo penguins, which visitors can see being hand-fed by their keepers.
There are 70 individual habitats, including a tropical lagoon, teeming with brightly-coloured tropical fish and rays. The biggest tank, the 10-metre-deep Endless Oceans exhibit, provides a home to sharks, rays and pair of rescued Loggerhead sea turtles.
Council leader Steve Brady said: "They consulted us and obviously we have an interest in the Deep, it was originally supported and financed by the council along with Lottery grants.
"They needed our permission to apply for the loan and obviously we support The Deep and are supportive of that.
"What's important is to fully support The Deep and make sure it survives."
The Deep has been approached for a comment.