FISH tanks can harbour a gastric bug capable of causing illness serious enough for infected children to have to go to hospital.
The bacterium, a strain of salmonella resistant to many drugs, is thought to have been imported from Asia with tropical fish.
Australian researchers proved the link between gastroenteritis and fish tanks by showing that the strains of salmonella in patients and in their home aquariums were genetically identical.
Diane Lightfoot, a salmonella specialist at the University of Melbourne and a member of the research team, said the study highlighted the need for care when cleaning tanks.
Fish were good pets, she said, "and fish tanks aren't to be feared. But commonsense hygiene is needed."
This included washing hands after touching the water or gravel and making sure the water did not splash onto surfaces where it could contaminate food, she said.
Dr Lightfoot began to suspect an association between tropical fish and the unusual variety of salmonella, known as Salmonella enterica serovar Paratyphi B, more than five years ago.
Between 10 and 20 of the 7000 salmonella infections reported each year in Australia were this variety. Multi-drug resistance was usually associated with overseas strains, but in most cases the infected Australians had not travelled recently.
Acting on the suspicion of British colleagues, she asked health authorities to see whether the people had aquariums. Most did, and samples were collected from some of the tanks.
The new genetic study, led by researchers at the NSW Department of Primary Industries and published in the journal Emerging Infectious Diseases, was the first definitive report showing that ornamental aquariums were a reservoir of the multidrug-resistant salmonella strain, Dr Lightfoot said.
It is thought to have arisen in South-East Asia, where antibiotics are widely used in fish aquaculture.
The vice-president of the Marine Aquarium Society of Sydney, Caevan Sachinwalla, washes his hands before putting them in his spectacular reef aquarium to protect the fish, corals and anemones. "There is satisfaction in being able to grow corals and other life forms and see them prosper," he said.