Primavera's matrix contained ... stories. Iggy had likened them to silent films, captivating and horrible, and he could watch them through her umbilicus. Where Primavera came from, people used dreamscapers and virtual reality tanks to escape their dreary lives for a little while. Primavera was a dreamscaper unto herself.
This world, where Hogwarts was, didn't have anything near the kind of technology Primavera's did. It did have things she could adapt to her own purposes. A
sensory deprivation tank plus the workings of her own matrix worked as well as any dreamscaper would have done for the aitch-men Primavera despised.
However, Primavera had a short attention span, easily grew tired of new toys. After she'd had it for about two weeks, the
tank sat unused in a corner of her room, acquiring an armour of discarded clothes and other things Primavera tossed over it or piled on it. Gogo had tried it once and deemed it "boring." It languished forgotten.
Gogo remembered it, though, when she saw the calming effect that floating in the prefects' bath had on Nny, and came to ask Primavera for its loan. Prima went her one better. "That thing? I don't care what you do with it. You can keep it!"
So, a troop of house-elves brought the isolation tank ("not so much
sensory deprivation as
stimulus reduction") from Primavera's room to Nny's. They hoped they could avoid bumping into anything. They'd like to keep their ears.