We have found a little sand in our pool just about every day. Not a lot, but maybe a 1/2 teaspoon. I have not vacuumed it yet (we use our Polaris all the time) and maybe after I vaccuum it will be gone (wishful thinking I suppose) but what could cause sand to come in from our filter? This is our 6th year with the pool and we have never had a problem like this before. 18 foot above ground. Thanks!