Jennifer,

The ERTZ family you describe apparently lived in Viola / Sac County / IA in 1895 (per the 1895 Iowa State Census index):

Henry Ertz 32 Wis W Sac Viola [birth year c1863]
Theresia Ertz 25 Wis W Sac Viola
Erva [sic] Ertz 4 Wis W Sac Viola
Angusta [sic] Ertz 2 Wis W Sac Viola
Ida Ertz 0 Sac W Sac Viola

The family appears to have moved to Carroll County by 1900; note the presence of Joseph NABERHAUS in the household, as it is probably relevant to the information that follows the census data.

1900 US Census, IA, Carroll Co., Kniest, ED 45, Sheet 5A
ERZ, John, head, June 1862, 37, married 10 years, WI GER GER, Farmer, rent farm
Theresia, wife, Apr 1870, married 10 years, bore 5 children/5 living, WI GER IA
Emma, daughter, Oct 1890, 9, WI
Augusta, daughter, Jan 1892, 8, WI
Ida, daughter, Dec 1894, 5, IA
Lilly, daughter, March 1897, 3, IA
Edward, son, Apr 1899, 1, IA
NABERHAUS, Joseph, servant, Sep 1878, 21, IA GER GER, Farm laborer

(A female child was born in Lancaster/Grant/WI on 1890-Oct-21 to Henry ERTZ and Theresa SHULTE. This was apparently Emma's birth.)

The Iowa Cemetery Records index confirms that Theresia died in 1901:
Theresa Ertz
born 18 Apr 1870
died 04 Aug 1901
buried Our Lady of Mt. Carmel in Kniest (Carroll County)

[No burial is listed for John or Henry ERTZ.]

I found the following interesting item in the Davenport Daily Republican, which suggests that [Henry John] ERTZ didn't die in 1901, but that the father was in legal trouble:

ERTZ Bound Over
Carroll, Nov. 15.--Henry ERTZ was arrested and brought to this city charged with attempting to poison the NABERHAUS family at BREDA last Sunday. On preliminary hearing he waived examination and was held to await action of the grand jury. ERTZ and NABERHAUS married sisters and considerable bitterness is said to have existed on account of family troubles. Both men are prominent in the community.
[Source: Davenport Daily Republican (Iowa), Nov 16 1901, p. 2]