Not that 6 hours ia a long time for a well maintained tank, but if the cans have not been cleaned out lately, if there is a lot of gunk inside, without flow, that gunk can go anaerobic very quickly, and "if" it did, once the power came back on, could have burped out a slug of toxic stuff (H2SO4 was already suggested) that may have been lethal.
This is one of the reasons I stopped using cans years ago. Power outages were common, and I noticed the media in cans would quickly begin smell like rotten eggs (H2SO4) if the flow was not maintained, so during any extended outage, I would pull the cans part, and toss media in an open shallow container of clean tank water and rinse out any buildup to help prevent the media from going anaerobic.
Although gunk is sometimes visible, H2SO4 itself is colorless.
You mention the Central American fish were unaffected, which seems reasonable because they adapted to often fluctuating conditions in nature, whereas rift lake species come from very stable conditions in nature.