Matlock Bath Aquarium sits in one of those buildings that seems, from the street, to promise very little and then proceeds to unfold room by room with the peculiar confidence of an old provincial attraction that has survived by refusing to tidy itself into blandness. In the hot early summer weather, while waiting for the gardens and the Illuminations to open, it made a useful pause from the glare of Matlock Bath’s pavements, the traffic, the chip shops, the river crowds and the steep limestone valley rising beyond the road towards High Tor.

Inside, the place has the character of a small cabinet of survivals, with aquarium tanks, rocks, gems, holographic pictures, mirrors and the faintly theatrical oddness of a tunnel of lights, all packed more tightly than expected into a building that keeps revealing another corner. It is not large, and it would be unfair to pretend otherwise, but for the modest entry price it has the virtue of variety, particularly for young children, who are often less interested in scale than in sequence: fish, glass, colour, reflection, water, stone, another doorway, another surprise.

The real centre of Matlock Bath Aquarium, however, is the old thermal pool, where the building’s earlier life suddenly becomes visible. This was once part of the bathing culture that helped make Matlock Bath a spa settlement, and the site opened as a swimming pool in the 1880s before being associated with hydropathic treatment, when visitors came not only for scenery but for the supposed benefit of warm mineral water. That same spring-fed water still moves through the pool, no longer serving bathers but giant koi and carp, which gather heavily and beautifully in the pale water beneath the old structure.

There is something quietly comic and rather wonderful about feeding koi from a gumball machine in a former Victorian swimming bath, as though the solemn rituals of health, class and improvement had been replaced by a handful of pellets and the open mouths of fish. Yet the change is not a betrayal of the place so much as another form of adaptation, for the water is still the working element here, rising warm from below, carrying its minerals, keeping the pool in use, and reminding visitors that Matlock Bath’s appeal has always depended on geology as much as scenery.

The petrifying well nearby adds another layer to that story, showing how mineral-rich water can slowly coat objects in stone-like deposits, turning natural chemistry into a minor theatre of patience. Between the carp pool and the well, the aquarium becomes less a conventional attraction than a small lesson in water’s persistence: water as cure, water as display, water as habitat, water as local industry, water as the reason people came here at all.

Outside, Matlock Bath resumes its louder rhythm, with the A6, the river, the amusements, the gardens and High Tor across the way, but the aquarium leaves behind a stranger impression than expected. It is compact, eccentric and slightly improbable, yet it preserves one of the town’s older functions with unusual directness: the spring still rises, the pool still holds life, and visitors still gather round the water.