Also, anything to actually do. Animals go crazy from extreme boredom in the same way that humans do, and in many cases with similar symptoms (self-harm, repetitive behavior, meaningless aggression).
Take exactly the same experiment but add as little as just a bunch of wheels for the mice to run on and I'm certain the results would be healthier.
They also had an environment which was not at all what their evolution had shaped them for. 0 gallons of water will kill you, 10.000 gallons of water will also kill you.
I'm pretty certain that fifticon was using water as an example and metaphor, not a specific element of the mouse Universe experiments.
Mice's evolutionary history has evolved them as a small, vulnerable, prey species. In the absence of predation pressures, rather than evolve to a well-balanced and well-functioning society, the same adaptations which are beneficial where most juveniles (and many adults) are killed by predators --- high sex drive, high fecundity, extensive foraging, occasional conflict among mice --- become pathological of themselves.
Too little water kills. Too much water kills.
To much predation destroys mice communities. Too little predation destroys mice communities.
And by extension, similar lifting of long-established, evolutionarily-shaping constraints might affect other populations negatively. The implications for human populations, whether localised (e.g., urban crowding) or global (overpopulation, resource conflict) are pretty clearly indicated in Calhoun's work.
To offer another possible analogy, beavers with nothing to gnaw on have dental abnormalities that eventually cause them to starve to death.
Did the lack of adversity and hardship cause them to fall into fatalistic fugues? Nah, It's just an unmet physiological need that isn't generalizable to humans.
One female mouse will give birth to six pups at once on average. They are unable to avoid the population bomb scenario. On the other hand, a human female rarely gives birth to more than one baby at a time, and we have contraceptives. So in this metaphor, mice are compulsive water drinkers, while humans can just drink one glass and leave.
Take exactly the same experiment but add as little as just a bunch of wheels for the mice to run on and I'm certain the results would be healthier.