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.
