Excellent condescention, thanks.
DO would be used by aerobic bacteria nitrifying/nitrafying the organics... Mostly nitrates being the result. Once the oxygen runs out, Anaerobic Nitrate Respiration then Anaerobic Denitrificaction (which I believe can occur via more than one pathway - one w/no sulfur) can reduce nitrate to gaseous N2.
My point is that there is a whole methodology of using Deep Sand Beds to detoxify aquariums, and I think the rumours of the release of toxic compounds are pretty exaggerated. That seems relevant.