Try putting a layer of floss under the bottom tray in your filter.
There is normally a void about 1/2 inch deep under the last tray.
Small particles of sand dust etc can escape through this void.
You could also attach a fine sponge pre filter to the intake on your external filter intake. This will need to be cleaned regularly but will help control the particles interning your filter just to be pushed back through into the tank.
Also make sure you vacuum the substrate with a gravel syphon during water changes.
There is normally a void about 1/2 inch deep under the last tray.
Small particles of sand dust etc can escape through this void.
You could also attach a fine sponge pre filter to the intake on your external filter intake. This will need to be cleaned regularly but will help control the particles interning your filter just to be pushed back through into the tank.
Also make sure you vacuum the substrate with a gravel syphon during water changes.

