Conservationists have expressed concern after more than a third of the creatures at a Sea Life centre died in a single year.
There were 812 deaths out of 2,293 creatures at Great Yarmouth Sea Life centre in 2015-16, a BBC investigation found.
The Marine Conservation Society (MCS), which has a sponsorship deal with Sea Life, said the figures were "a cause for concern".
The MCS said mortality rates of sharks and rays were particularly worrying - more than a fifth died at Great Yarmouth, which is almost double the average.
Figures across eight Sea Life centres were obtained through a series of Freedom of Information requests by the BBC, with a total of 4,500 creatures dying in 2015-16.
The deaths were at centres in Birmingham, Blackpool, Great Yarmouth, Hunstanton, London, Manchester, Scarborough and Weymouth. No data was provided for Brighton's centre.
Dr Ben Garrod, MCS Ocean Ambassador, said the mortality figures are not acceptable.
"You wouldn't go into hospital and expect a one in three chance of dying," he said. "You wouldn't expect that in a zoo. I don't think it is acceptable."
Sea Life owner Merlin Entertainments attributed some losses at its Great Yarmouth centre in 2016 to a "technically complex" water problem which had since been resolved.
The MCS added that mortality figures included short-lived species such as jellyfish, which live for a matter of months.
Dr Chris Tuckett, MCS head of programmes said: "The response we've seen from Sea Life reassures us to some degree.
"In particular, the incidents of high mortality appear to be the result of unforeseen problems which the chain has taken steps to rectify.
"There are still questions over the keeping of some animals, including endangered species, and we would like to see a more robust set of record-keeping maintained by public aquariums to demonstrate that they give the specimens in their tanks care and attention as a matter of routine".
MCS has accepted sponsorship for its Pocket Good Fish Guide from Sea Life.
Dr Tuckett said MCS will have further discussions with Sea Life if future collaboration is to be pursued.
A Sea Life spokesman said: "Sea Life places the very highest priority on the welfare of the animals at its sites, which are supported by experienced animal care teams, marine biologists and world-renowned veterinary consultants.
"Sea Life has a strong track record in animal husbandry and complies with all the laws and regulations under the Zoo Licensing Act.
"Furthermore, we take an active role in promoting conservation, playing a leading part in rescue and rehabilitation, breeding select species at our facilities and educating the many millions of visitors to our attractions."