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Plant substrate mystery


Need help here. I have an aquarium friend with a peculiar problem.  After a
20% water change has been done on two of her tanks with plants, everything
dies.  The water changes are every two weeks and the gravel is vaccumed in
the process.  Here are the details:

120g/40g (breeder)
white silica sand substrate, 1 inch thick
low current/filtration
shell dwellers and other cichlids

12/24 hours after sand vaccum
        the fish are seen at the surface gasping for air (possible O2 depletion)
        surface scum/film

pH 7.6
Amm/Nitrite 0
Nitrate 20ppm
no chloramines

What causes the fish to die?

Several possibilities:
        Activation of dormant, good bacteria that remove all of the O2
        Anaerobic bacteria are stirred up and deplete the O2 somehow (not
sure about how this would    work)
        Phosphates (level not known) somehow fit in the picture


Thanks in advance.

Matthew T. Mason                            "The young do not know
Doctoral Student                                  enough to be prudent,
Department of Molecular Genetics      and therefore they attempt
The Ohio State University                   the impossible -- and
mason.163 at osu_edu                          achieve it, generation
                                                            after generation."
                                                            -- Pearl S. Buck