Lahnfenster, Water observation center in Gießen, Germany.

The Lahnfenster is a partly underground river information centre run by the Regierungspräsidium Gießen at the weir of the Klinkel'sche Mühle on the Lahn. Its lower room, about 40 square metres, has three-square-metre glass panes facing both the fish passage and the river itself, plus a shallow-water zone; above it stand a viewing platform and a pavilion with an exhibition and teaching room.

In 2007 the city of Gießen opened a fish pass at the Klinkel'sche Mühle weir, and the Upper Fisheries Authority had a 16-square-metre underground viewing room built at its upper outlet. After the room proved too small for school groups, it was rebuilt for around 840,000 euros and reopened on 9 June 2014, three months before the Landesgartenschau.

The Lahnfenster is run by the Regierungspräsidium Gießen as an environmental education site outside school, used by families, interested groups and individual visitors. School classes made up 4,000 of the 20,000 visitors counted in the first five years. Three webcams record the fish, and the best footage is published on the authority's YouTube channel and Facebook page.

The underground room is reached by a staircase and, since the 2013–2014 rebuild, by a ramp from the bridge over the weir, so both riverbanks connect barrier-free. Entry is free. The room measures about 40 square metres, with three-square-metre panes facing the fish passage and the Lahn itself; groups of schoolchildren once overcrowded the old 16-square-metre space.

The fish pass was commissioned in 2007 so species such as perch, gudgeon, chub, brown trout, roach and barbel could climb the 2.30-metre difference at the Klinkel'sche Mühle weir. The viewing room at its upper mouth measures 16 square metres; after the 2014 rebuild it grew to about 40 square metres, and 38,000 people came within three months.