We just installed a 24' pool package that came with pump, sand filter, etc. Anyhow, everytime I vacuum the pool, the small flies are coming back into the pool. The sand filter is a 19" and came with 100# of sand (filter says it holds 150#). I have the filter set at filter, but don't understand why something that size goes thru. Could it be there's not enough sand, or is something else I could be doing wrong.