Apple snails are easy to control. They lay big clusters of eggs above the water line where you can easily find and remove them. Or should be easy, sounds like you might have missed that.
If you do indeed have apple snails and take the eggs off for population control, be sure to dispose of the eggs properly. Drowning them works well.
They are taking over in south Louisiana. http://www.louisianasportsman.com/details.php?id=9404