If the crack is on a bottom corner, odds are any type repair will be futile, due to the tank flexing when full and empty. But, it does not hurt to try. Make sure the area is CLEAN and DRY. Use sandpaper to rough up the ABS. Then apply the plastic glue material WITH a small piece of standard fiberglass screen material to strengthen the repair and prevent the tank from flexing. Is this a CRACK? If so, you will want to drill a small hole at the end of each end of the crack to prevent the crack from continuing. Doug