I would think that pool water being at 3 parts per MILLION isn't going to be a problem, doubt you'll get results anything close to the coke/mentos geyser!
They likely mean a typical household cleaning mix of bleach and water which would likely bring the concentration up to 100-500 ppm - a BIG difference.

BTW - I will be trying this to clean that darker line left over from wintering this weekend, it is below teh water line, so it may be touch and go for me - wish me luck