Whitespot is caused by a tiny little bug that burrows into your fish. The white spot you see is the egg capsule from the tiny bug. This will eventually drop off and these eggs will hatch. Then millions more of those bugs will be in your tank attacking your fish producing more white spot. For this bug to get into your tank you must have introduced it somehow. Do you quarantine all new fish? Do you sterilise everything that comes into contact with other fish/aquaria before touching other tanks or equipment? If not, then one of those actions will be how the white spot got into your tank.