A total of 48 people, including 34 children, have so far been hospitalized. The youngest of the victims is nine months old.
Visitors to the Waterville aquapark in Russia's second-largest city started experiencing health problems, including skin complaints, on Wednesday evening. It is believed that the swimming pools had been over-chlorinated.
The poisoning victims said they would be seeking compensation.
Prosecutors have launched a probe into a breach of sanitary regulations leading to mass poisoning cases, and for failure to meet safety standards. The first offence is punishable by a three-year prison term.
On Wednesday 2,270 people visited the waterpark. The center has been temporarily closed and water samples have been taken.
A spokesman for the aquapark said however, only one person needed treatment for an eye irritation and water tests carried out every three hours showed no breaches of regulations.
The worst disaster to hit a Russian aquapark was in 2004, when the roof of the Transvaal aquapark in Moscow collapsed, killing 28 people and injuring over 100.
