Most likely hydrogen sulfide gas from anaerobic bacteria being released when the substrate is stirred up. The fish can suddenly die. With denitrification occurring the water parameters can actually look pretty good, but death can be lurking in the substrate. If regular tank maintenance is an issue for you perhaps you should consider removing the substrate or sand.
There are lots of articles on hydrogen sulfide poisoning. Here are two:
http://www.aquarium-pond-answers.com/2008/11/hydrogen-sulfides.html
http://www.fishyportal.com/cgi-bin/pub/diag?c=v&id=60
There are lots of articles on hydrogen sulfide poisoning. Here are two:
http://www.aquarium-pond-answers.com/2008/11/hydrogen-sulfides.html
http://www.fishyportal.com/cgi-bin/pub/diag?c=v&id=60
