In a statement, the Peru team says, "We are profoundly sorry for the incident, which affects not only the athlete but all Peruvians."
Fiol says, "I don't understand how this could have happened. I've always promoted fair play."
Fiol has the option of asking for the result of the B sample.
Canada could benefit if Fiol is stripped of his medal as Zack Chertat's bronze would be upgraded to silver while teammate Alec Page would move up to third place.
Page called his fourth-place finish "bittersweet" but said he wasn't going to get ahead of himself.
"I would be happy, but I'm going to hold back for now, just because nothing is official yet," Page said at the pool Thursday night.
— With files from The Associated Press