Tampere town environmental engineer Paula Saxholm said that it was still inconclusive till Monday why the quality of the lake's water has deteriorated.
Preliminary results from the water sample taken from the lake will be available on Tuesday.
"Blue-green algae has not been detected in the lake," said Saxholm.
Those who have experienced stomach symptoms have recounted knowing other people who became ill after swimming in the lake, said Saxholm.
According to Saxholm, those who went out swimming apparently began to vomit after swimming.
The town authorities adviced residents to avoid swimming along the beaches of the lake in Ikuri and Ristimäki.
