As sylvia said either get your own test kit or, when you take a sample to the lfs, dont settle for "its perfect" ask what the exact readings are.
What make of filter do you use? If its one that has various media types can you get hold of polyester pads? these 'polish' the water of small particals that otherwise get through.
One more point, do you use an airstone or venturi to add bubbles to the tank? only reason I ask is that it can result in tiny bubbles that stay submerged for a long time and seem to be particals.
Jon