>>was that the first time you'd moved the gravel in a while? I'll usually stir up the substrata a bit every day or 2, just to keep toxin pockets from forming.<<
Toxin - what does that look like? I had noticed some white feathery stuff on the driftwood and in the back corner of the tank (near the intake vent) - is that toxin? I wonder if that's what killed my Cory cat and Yoyo Loach since they are both bottom feeders - maybe they ate some of the toxin?
Also, we are having the same issue with water cloudiness - we cycled the tank back in December and the fish were all fine. Then, after 30 days (with fish) we did a full water change (which I was later told may have been the problem). The water was crystal clear the first few days but then became progressively cloudy each day. Could it have been the full water change? We have a 14 gal biocube, with the bioballs (which we did NOT clean).