The dead snails are adults. Here's an image of the clump of them:
http://gallery.nanfa...snails.jpg.html
Their deaths happened at the same time that the water was allowed to get much cloudier than it usually is. I noticed it getting worse and worse but wasn't able to do a water change for a few days. When I finally did a nearly full water change on Wednesday, that was when I noticed the dead snails. It was pretty foul; the water actually had an odor and was so opaque that you couldn't see the plants at all. That's why I think it was the potting mix that killed the snails. The tank without potting mix doesn't get cloudy and smelly like this one does. It also doesn't volcano up with sulfurous gases, which is what the Elassoma gilberti tank was doing on Tuesday.
I think the reason that the snails died is because this is the first cloudy water incident that has gotten to the point of smelling since way back a couple months ago when all the snails died off. I guess the snails that survived and bred passed on resistance to most of their offspring (not all of the snails died this time) but some of the offspring were still sensitive. So the next time that cloudy gross water happened, the sensitive snails succumbed.
*nods* It makes sense now. I hadn't put the timing of the cloudy water and the appearance of the dead snails together yet, but talking about it to you all has helped me figure out their cause of death. I had been thinking about a couple other things that might have caused it (new dechlorinator, rotting bloodworms, media from microworm culture) but now that I think about it, the rotting sulfurous stuff from the potting mix does make the most sense.
I just noticed the fin color this morning. It either suddenly occurred or I didn't notice it before today.
Edited by EricaWieser, 17 November 2011 - 01:00 PM.