So you have a snail problem do you?

Remember that everything must have source of food if it is to survive, however you appear to be cleaning the tank properly so there shouldn't be any gunk or uneaten food left. Do you have any live plants in your tank or have you added any recently? Live plants are usually the starting point for snail problems as they offer a 'happy breeding ground' for the snails. You may have seen small, clear sacks on the aquarium glass, these are the eggs sacs.

What I would do is use a bit of elbow grease and remove as many snails as possible by hand, also crush any egg sacks you can find. This isn't going to sort the problem out completely however it will give you a better footing.

If this doesn't work then it may be a case of stripping down the tank and zapping the little termites!