Those things don't usually get measured in microns, there's just a screen in there to catch large debris. In reality, if it gets by the screen in the pickup its likely going to get by the screen in those filters also. A good filter ahead of the pump - measured in microns - would have to be about as large as a regular filter so as not to restrict flow. And if you were to find one that large then the one after the pump would be pointless.