A septic tank, which may be converted into a grease trap. The tank is preferably made of plastic reinforced with fiber glass and it is further reinforced due to the construction. The tank is light weight, therefore easy to install, is durable and resistant to corrosion due to ground moisture, and will not deteriorate because of exposure to acids and alkalies. The cover of the present tank interfits with the tank in fluid tight relation. It is readily converted into a grease trap by the addition of an extension onto the outlet pipe which extends downward into the tank.