Built environment

We learn more and more every day about the bacteria, viruses, and fungi that live within us and support us. It's our own personal world: the microbiome. And researchers have identified ecologic niches with varying microbial occupants. And it seems that cities have unique microbiomes, too.
Fish + bacteria = 25% mortality. Fish + bacteria + rocks = 5% mortality. What magic lies within the rocks? What can farm-raised fish teach us?