I read this paper, and wanted to add some 0,03$, based on behavior of wild common hippos:
1. Space for night activity, night foraging and night enrichment.
Common hippos in the wild leave water, and travel individually up to >10km inland to graze singly or in female-calf pairs. This is their main physical activity. Closing hippos in small stables at night is not good.
Zoos should give hippos sizeable land area with access at night. Hippos should be given food to feed at night, and enrichment. Letting animals distance themselves from each other then is better.
Some solutions might include installing weak source of light if there is a fear of zoo hippos falling into moats etc and providing night access to enclosures of other large animals, eg. antelope, zebra or giraffe.
2. Social grouping: females socially, males socially or can be singly.
A proportion of wild male hippos are chased away and forced to live singly, often in very small pools or swamps away from rivers and main hippo herds. It can be argued that male hippos can be kept singly because it is found in the wild. Of course, here is somewhat of an argument if these males are not stressed in the wild, and that wild conditions do not equal good conditions.
3. Soft surface, obesity and foot problems.
They are problem of zoo elephants and rhinos. They may be problem for zoo hippos, too. This might be researched
4. Deep water - hippos probably do not need it.
Wild hippos prefer shallow water just enough to submerge. They are not buoyant and do not swim if they can avoid to, moving by walking on the bottom.
5. Flat surface and easy way out of water - hippos do not need it.
Wild hippos are unexpectedly good climbers. Herds living in rivers with steep banks or canyons eg. Murchison Falls National Park, Uganda, commute daily directly up and down high and very steep slopes with 45 to 60 degrees grade. Hippos make paths going normally directly up and down slope, steep enough to force man to climb on all fours.
BTW, climbing such a hippo path in shrubbery is dangerous - if there is a hippo, it will run over you like a living truck.