I have a problem with compost flies in my houseplants. According to a gardening show I saw, they can be discouraged from laying eggs in potting soil by topdressing the soil with washed sand. On the show, they used bagged "washed sand" by Whitney Farms. I can't find sand that specifies it is washed, and the folks at Home Depot told me sandbox sand would suit my purpose. My concern is that if there is salt in the sand it will kill my plants. Comments?