this is an extremely common question and I am quite confident in saying that they are more than likely something called planeria. What are planeria? Well basically they are free living parasites that get into your tank on live plants etc. they are not known to cause any harm to your fish. Many people who have these little critters tend to overfeed their fish,or leave uneaten food in the tank. I don't think there is a chemical that I could recommend 100% eradicating them. normally reduced feeding over a period of weeks can help reduce their numbers, manually harvesting them is also another way of reducing numbers. Other than that, the only other surefire way of getting rid of them is completely stripping your tank down, including filtration. Not something that you really want to do.
