According to a death log obtained by the Oregonian, the deceased animals included sea horses, bamboo sharks, sea stars and fish. Causes of death included starvation, infection and animal-on-animal attacks.
"The death log submitted appears to be fabricated, or to be skewed by such deaths as dozens of snails, baby damsel fish and others," the aquarium's co-owner, Vince Covino, said in a statement.
"It's defamatory to our highly qualified team of marine biologists," Covino said.
The news of the deaths upset many of the aquarium's customers, including Kimber Rhoades, who was visiting the aquarium Tuesday.
"It's a little sad to read," Rhoades said. "Portland needs more things for children to do. We're hoping it's just a wake-up call for them to maybe just do what they need to do to keep business open."
In February, Ammon Covino, who co-owns the aquarium with his brother, was arrested for illegally purchasing marine life in Florida and shipping it without a permit to another aquarium the brothers own in Boise, ID.
The Portland Aquarium opened in December of last year. The death log was from February 18 to May 16, 2013.
KGW Reporter Katherine Cook contributed to this report.