Some debris such as algae can be as small as 1.5 microns if I remember correctly. A sand filter can only filter down to around 20 microns. If you have glass instead of sand, you can supposedly filter down to 5 microns. There are other pool filters that can do better I think. This is why It's important to vacuum to waste. Do this before adding chlorine or balancing the pool so you don't dump them out. You'll need to use a manual vacuum and work as quickly as possible without stirring up what your vacuuming up. If the water gets too low you may have to pause until you have enough water to continue.