This was at the end of the original Pachyderm House, past the elephant stalls. When the Elephant House was re-modelled with separate accommodation for the bull elephant, this was in the area that had been for the hippos. They had a small pool near the public end of the area.
The next time I visited the Zoo, around 1993, I think, the hippos were gone but a sign stated that the Zoo were acquiring a pair of Pygmy Hippos.