I would agree with your parents as Carl and Augusta.
I tracked them from 1860 Wisconsin to 1870 and 1880 Goodhue County, MN. In 1885, Frank has married Clara, and has a son Eddie 1, and daughter Linda, 1 mo old. Living with them is Augusta Fisher, his sister on the 1870 census.

I would suggest checking to see if Carl/Charles Fisher or Augusta Fisher, both who died in Lac Qui Parle County, MN, left any probate documents that might identify Frank as their son with a location for him that matches up. Last census I find Augusta is 1905 in Lac Qui Parle in MN, then a widow. Carl was still alive in 1900.

I couldn't find Frank on 1900 and on census. What was your grandfather's name, year of birth and state of birth? Tracking him down on the census might help.

On MN death index site found - Lac Qui Parle County
Carl Fisher
Date of Death: 02 Dec 1904
Mrs Fisher (probably Augusta)
Date of Death: 02 May 1906

I'm not related.