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Septic tank aerators can be very expensive. They range anywhere from $200 to over $1000. This being the case most reputable aerator manufactures incorporated failure points to allow for easy replacement. Let me explain. Most septic aerators use diaphragms to pump the air. They are purposely designed to be the weakest point in the aerator. That way if something goes wrong with your aerator your diaphragms will break. This means that you will only have to replace the diaphragms and not the entire aerator.
Most rebuild kits come in 2 different options, Complete Kit, and Diaphragm Only. What is included in both options varies from kit to kit. However, unless you know with absolute certainty that the diaphragms are the only part that needs to be replace, it is highly recommended that you purchase the Complete Kit.