John and Mary Zell can be found in the 1930 census in Syracuse, Onondaga Co., NY; 1920 census in Rome, Oneida Co., NY: 1910 census Braceville Twp., Trumbull Co., OH; 1900 census in College Hill, Beaver Co., PA (but misindexed under the name Tell). Living next to them is Frank Zell, born in Feb 1872. In each census John's parents are listed as born in Germany. John and Mary were married ca1896.
The 1880 census for Pittsburgh, Allegheny Co., PA includes:
Henry Zell 35, born in Hesse Casse Margaret 28, born in Bavaria Mary E. 12 John A. 10 Francis 8 Philip 5 Cecelia 10/12
The 1870 census for East Birmingham (now part of Pittsburgh), Allegheny Co., PA:
Henry Zell 25, born in Germany Margaret 22, born in Germany Mary 2 Henry 1
The 1900 census for Beaver County also includes Margaret Zell, born May 1841, in Germany, age 51, immigrated 1851
Some of these records you can look at for free at labs.familysearch.org and Ancestry.com. For those you cannot see for free I suggest checking with your local libraries to see if they have Ancestry or Heritage Quest.
To confirm the lineage I have found I suggest getting John's death certificate and obituary. Mary's obit and death certificate should have her parents. Don't be surprised if her parents were Evan and Kate Pugh (1880 census Beaver County). More than likely John and Mary got their marriage license at the Beaver County Register of Wills Office. It may have info on parents.