I see this thread is focussed on river hippos, and I have not seen enough of them in the UK to comment. However, Pygmy hippos have a similar issue. Being essentially nocturnal forest animals, my ideal pygmy hippo exhibit would be an aquatic area with suitable viewing with access to a reasonable land area either heavily planted or just an enclosed area of woodland. Are there any exhibits people know of like that? I know Marwell has a large outside area but as I recall it was basically a field - not ideal for a forest animal.