The fish were part of major exhibition of conceptual art at Tate Modern and their deaths have left the gallery facing accusations of neglect.
A total of 55 fish were kept in three tanks as part of an installation by Cildo Meireles, a prominent Brazilian artist, but almost a quarter of them died during the 13 week long exhibition.
The fish were Ghost Catfish and Chanda Ranga - commonly known as Glass fish - and were used in two art works, entitled Atraves or Through, and Red Shift.
Glass fish, which naturally live in fresh or brackish water in Burma, India and Thailand, usually live for between two and three years. Ghost Catfish, which have translucent bodies allowing onlookers to see their inner bodies, are expected to live for between three and five years.
Animal rights organisations have condemned the use of fish in the exhibition.
A spokesman for the RSPCA, said: "The RSPCA opposes any so called 'art exhibition' that causes animals of any species distress or suffering. The RSPCA believes that fish are capable of feeling pain and suffering, they therefore should be treated with respect and in a manner that minimises the risk of harm.
"We are concerned about the deaths of the 12 fish in this instance. If this art exhibition involved dogs, then there would be a national outcry, why should the use of fish be any different?"
A spokesman for the animal rights organisation, People for the Ethical Treatment of Animals (PETA), said: "It is time for the Tate Modern and other galleries to take a stand against the use of animals in so-called 'art' and to refuse to celebrate the exploitation of these sentient beings for frivolous reasons.
"Tropical fish, who were born to forage among brilliantly coloured coral reefs, belong in the deep blue depths of the sea, not suffering a miserable existence in glass tanks in art galleries so that people can gawp at them."
A spokesman for Tate, said: "The fish are an integral part of these artworks. Tate sourced the fish from a reputable aquarium specialist who advised on appropriate conditions to ensure their welfare. Tate carefully maintained these conditions throughout the exhibition."
Mr Meireles was unavailable for comment. In an introduction to the exhibition, he explained his work by saying: "My work aspires to a condition of density, great simplicity, directness, openness of language and interaction."
It is not the first time that fish have suffered for art. In 2000, the Chilean artist Marco Evaristti sparked outrage for a work he exhibited at the Trapholt Art Museum in Denmark. The display, entitled Helena, featured 10 blenders containing goldfish. Evaristti said that he wanted people "to do battle with their conscience" so visitors to the exhibition were invited to turn on the blenders. Several of the fish were liquidised which led to the museum director, being charged with, but later acquitted, of animal cruelty.
The artist Damien Hirst also caused controversy by using animals in his work. In 1995, his work Mother and Child Divided, a cow and a calf sliced in half and displayed in glass tanks filled with formaldehyde, won the Turner prize, despite protest from animal rights organisations. He also created a similar work using a shark.
The exhibition of Cildo Meireles' work ran from Oct 14, 2008, to Jan 11, 2009.