It is out-dated, but the zoo wants now to build a new house for hippos in the next years. But is it really necessary to keept hippos also in Frankfurt ? The Opel-Zoo Kronberg keeps them too, and this zoo is very close to Frankfurt, a lot of the Frannkfurt people visits both zoos, because Kronberg does have elephants and Frankfurt not.
So I would keep rhinos at Frankfurt and Hippos at Kronberg, but there is no friendship between both zoos.