I posted a couple of weeks ago about mysterious pygmy cory deaths. I lost 3 out of 6 shortly after I got them. I saw no obvious cause of death, and everything has been fine for about 2 weeks now. Just a few minutes ago I looked in the tank and one had its head buried in the gravel. It was face down in a gap between a rock and a plant, with its tail sticking up. I was worried, so I touched it with my net, and it swam a few inches then rested on the bottom, and was breathing heavily. I watched for a minute or two, and then it started darting around the tank before finally trying to wedge itself under one of the decorations I have. The other two are acting normally, and the one in question seemed fine about an hour ago.

I just did a water change, and the water is fine. 0 ammonia, 0 nitrite, <5 nitrate, pH 7.3. The two biggest changes recently have been a DIY CO2 system a few days ago, but I have lots of plants, so that shouldn't be a problem, and some root fertilizer tabs this morning.

Is there something else I should check for? What is happening?