Earlier this week, keepers noticed that Kita, a 15-year-old female, was lethargic and not eating.
Veterinarians at the zoo's animal health centre extracted more than a litre of fluid from the tiger's chest cavity, raising concerns that she had cancer, the zoo said in statement.
Preliminary blood tests did not indicate Kita is suffering from cancer.
Kita was put back in the tiger enclosure Thursday, and her appetite has returned. Animal care staff are continuing to monitor her closely.
It is not known what caused the infection but it is not contagious, officials added.
"Kita normally shows a dislike for the veterinary staff, which we take with good humour, and when she was not her feisty self when seeing me, I knew something wasn't right," said Dr. Doug Whiteside, the zoo's senior veterinarian.
Kita has had many health problems — stemming from a nervous disorder — since arriving from the Toronto Zoo in 1996.
Kita gave birth to four cubs 10 years ago, with three of them moving to other zoos.
One of the females, Katja, is still at the Calgary Zoo. In 2007, Kita gave birth to a male, Vitali, who also remains in Calgary.
