Murky/Green water is another form of algae that is made up of really fine particles that a filter cannot pick up. The fool-proof way of stopping this from happening, to give you that nice crystal clear water, is to put a UV light inline with your pump. The UV light drops these particles out of suspension so that the filter can then pick them up. These products below are the short term fix for anyone not wanting to put a UV light on their pond and will generally get you the same result, but the murky/green water may or may not return.