Of the original three springs, only the Ottilienquelle was still used for spa purposes. A brochure printed around 1900 praised the numerous advantages of the sanatorium. As the „first and most outstanding special institution for asthma in Germany“, it offered therapies for several lung diseases such as asthma, emphysema, chronic bronchial catarrh and residual lung and pleurisy infections. The then „fashionable disease“ nervousness as a result of mental overwork and in the form of functional neuroses, neurasthenia or hysteria could also be alleviated. Furthermore, women’s disorders, heart and circulatory diseases, metabolic diseases such as diabetes, obesity or anaemia, digestive problems, malfunctions of the bladder and kidneys were on the treatment plan.
The holistic approach to therapy also gave people in general need of recuperation the prospect of a successful stay at the spa. Since the end of the 19th century, the sanatorium no longer offered inhalation cures; they were replaced by therapies in the brine vapour room. From 1903, no cases of open TB were treated either. Despite all her efforts, Gertrud Fischer was not able to achieve lasting economic success. This is indicated, among other things, by the reference in the 1903 paper written by the „directing physician“ Dr. Heim on the occasion of the 25th anniversary to the urgent need to present the quality of the Inselbad to referring physicians and potential spa guests. As early as 31 July 1906, Gertrud Fischer was forced to sell the sanatorium to the entrepreneur Josef Postinet from Duisburg Ruhrort. Presumably due to financial constraints, the latter sold the site between the Pader and Rothe rivers to the district, which erected a building for a winter school there in 1908. However, the problems continued to grow. According to a decision by the district president on 18 July 1907, the „Curanstalt Inselbad“ was not granted a concession because of structural defects. Obviously, the financial hardships had not allowed further investments. But the owner, Josef Postinet, refused to accept the conditions imposed by the authorities. In addition, the Paderborn Kreissparkasse, as the holder of the first mortgage, applied for the sanatorium to be placed under receivership in view of a registered debt mountain of 300,000 M. Against this background, the previous owner Gertrud Fischer bought back the sanatorium in February 1908, willing to fulfil the official requirements. Officially, she resumed running the business on 1 July 1908.
In order to establish a solid business basis, Gertrud Fischer founded the „Ottilienquelle GmbH zu Paderborn (in Gründung begriffen)“ – „Ottilienquelle GmbH zu Paderborn (under incorporation)“ – on 28 September 1909. Its purpose was to expand the medicinal and table water business and to distribute it in Germany and abroad. In the investor brochure, the initiators emphasised the long tradition of the Ottilia spring and the high quality of its water. The spring would not be subject to the influence of rain or prolonged drought. The content of silicic acid in the spring water would be extraordinarily low, which indicated only few organic substances and thus a long shelf life of the mineral water. As a founding partner, Gertrud Fischer contributed a 2-hectare site with the Ottilia spring and the swimming pool, the main building, the machine building with a 15 hp steam engine, a hall with the filling equipment for 10,000 bottles per day, the bottle warehouse as well as 200,000 Mark to the company. The other shareholders were to contribute 400,000 Mark, take over the mortgages on the sanatorium and contribute 340,000 Mark to an operating fund. Allegedly, the annual profit to date was equivalent to an interest rate of 100,000 M. The target was the bottling and distribution of 5 million bottles per year. The maximum output was to be 8 million bottles and an annual profit of 150,000 M.
No precise figures are currently available on the actual development. Nevertheless, delivery warehouses in Soest, Werl, Delbrück and the documented direct supply to the Sauerland region in Arnsberg, Winterberg, Bestwig, Olsberg and Brilon indicate a supraregional sales market. After the filling plant was completely destroyed on 27 March 1945, the filling house was rebuilt by 1948 and operations resumed. Bottling and distribution of the water from the Ottilia spring was discontinued in 1965.
However, all these measures were not able to lead the Curanstalt Inselbad into calm economic waters. On 7 March 1912, Gertrud Fischer finally sold the business to the „Landrat“ (district administrator) von Laer, who then ordered the closure of the Inselbad Sanatorium in September of the same year.