Sand filters do let fine stuff thru. Some of us have been adding a little DE powder to the system to catch the smaller particles. It does work. Be careful and go slowly for the first couple times until you 'calibrate' how much DE to throw in. What I've done is to note the pressure on a clean filter, just after backwashing, then add a cup of DE to the skimmer and wait a while for the pressure to stabilize, at least 10 minutes. Add another cup and keep repeating the cycle until the pressure is about 1 PSI higher than when you started. Once you've done this a couple times, and you are 'calibrated' you can just throw in the right amount of DE after you backwash. DE should be mixed with water before chucking it in the skimmer. The DE forms a layer on top of the sand and is very efficient at trapping smaller particles. Be careful when vacuuming to keep an eye on the pressure, especially when you may have a bunch of dirt on the pool bottom. Pressure can rise rapidly. ALSO.....WHEN HANDLING DE POWDER BE CAREFUL NOT TO INHALE THE DUST.
Al