A tank adapter has a body with first and second ends and an axial hole. A hollow cylinder of the body has an external tapered thread from the first end toward the second end of the body. A flange between the first and second ends has a flat face extending radially outward from the hollow cylinder to a flange outer diameter. A gage mount portion has a gage mount outer diameter that is less than the flange outer diameter and extends from the second end toward the first end and has gage mounting features for securing a level gage. A gasket mates with the flat face. In a first configuration, the external tapered thread engages an internal tapered thread of a first tank opening. In a second configuration, a collar nut compresses the gasket between the flange and a top rim of a second tank opening.