The sources have little to report about the development during the early 1850s. Presumably, the bathing and healing business continued at a modest level. Eventually, however, a financially strong investor was found in the form of the Dortmund „Kommerzienrat“ Friedrich Wiesehahn. Apparently, several successful spa visits by his wife Ottilie, who was suffering from lung disease, had convinced him of the healing power of the springs and made him decide to acquire the Inselbad and develop it into a profitable spa – a widespread business idea at the time. Wiesehahn implemented his project in four steps.
Firstly, he acquired the property on 10 May 1856. In addition, he expanded the entire property to 16 acres by purchasing around 10 morgen (approx. 2.5 ha) of land, which was located between the Pader and the Rothe and belonged to the city of Paderborn. Wiesehahn had the old buildings demolished and a new spa house, a colonnade and a one-storey bathhouse built. Now it was also possible to administer warm baths and additional balneological measures, which was a necessary expansion of services in view of the competitors in Lippspringe, Meinberg and Driburg. In addition to the Ottilienquelle, the Marienquelle and the Badequelle were also used for the therapeutic operation.
Secondly, he hired qualified staff. With Johann Conrad Hörling (1819-1883) as the responsible spa doctor, the Curanstalt not only had an experienced physician, but also one who was active in public relations. Thirdly, Wiesehahn ensured efficient marketing of the medical and therapeutic qualities of his institution. In order to strengthen the level of awareness and the supraregional professional reputation, his administrator H. Becker commissioned the renowned Heidelberg chemist Professor Ludwig Carius with a new chemical analysis, which was published in 1866 in the leading professional journal of the time. Studies by the Leipzig chemist Ernst von Meyer in 1873 confirmed the outstanding quality of the Ottilia spring water. The fact that Wiesehahn named the central spring „Ottilienquelle“ in honour of his wife was by no means just an expression of personal affection. Economic marketing considerations probably also played a role, as the name evoked associations with the saint and healer Ottilie among his contemporaries. Fourthly, Wiesehahn ensured a varied spa programme with concerts, leisure activities and much more.
After about a year of renovation and expansion work, the official opening took place at the start of the season on 21 May 1857. In fact, the sanatorium attracted numerous guests from outside the region in the years to come, although the season remained limited to the months between May and September. However, the „Curanstalt Inselbad“ suffered considerably from the fact that, as an open facility, it was also accessible to the local population. The fun-loving visitors from Paderborn smoked, probably also drank alcohol, and bothered the spa guests from out of town, who were in need of rest and recuperation.Obviously, the conflicts increased and in the long run such conditions were detrimental to the smooth running of the spa. It can be assumed that Friedrich Wiesehahn sold the Inselbad to the timber merchant Bäumer from Lünen near Dortmund in 1876, not only for reasons of age but also because of the constant disputes with the people of Paderborn. This man in turn added a second storey to the previously single-storey bathhouse and built a concert hall in the park. Otherwise, however, he seems to have had little understanding of the balneological spa business. His successor, Dr. Brügelmann, complained about the disastrous structural condition of the facility, the unsuitable staff and the poor reputation as a sanatorium.