Re: Blanks in my family tree, HELP?

Alexander Linn, age 27, married Ida E. Fast, age 31, on 20 November 1884 at Brooke, Adams, Iowa.He was born in Washington County, Pennsylvania, to Andrew J. Linn and Harriett Merrin.She was born in Ashland County, Ohio, to George Fast and Elizabeth Heiffnier.The marriage record is here:
https://familysearch.org/pal:/MM9.1.1/XJ8S-14W

The 1900 census of New Market town, Taylor, Iowa shows the household:
Household Gender Age
HeadAlexander S LinnMborn August 1855
SpouseIda E LinnFborn September 1853
ChildPercival R LinnMborn September 1885
ChildEthel M LinnFborn March 1887
ChildMyrna I LinnFborn February 1889
ChildRay C LinnMborn December 1890
ChildElsie D LinnF born August 1893
The 1900 census record is here:
https://familysearch.org/pal:/MM9.1.1/M9LG-75G

Andrew J. Linn and Harriet Merier were married on 24 March 1853 in Morrow County, Ohio.The marriage record is here:
https://familysearch.org/pal:/MM9.3.1/TH-1-17962-27040-47?cc=1614804

Andrew and Harriet Linn show up in only one census, in 1910 at Nodaway, Adams, Iowa, where we learn that he had been born in Pennsylvania in approximately 1828, and she had been born in New Jersey, approximately 1833.
The census record is here:
https://familysearch.org/pal:/MM9.1.1/MLB1-7QV