Hmm, I've been looking all over for an answer to this question. Maybe someone here can help me. I have Malaysian Trumpet Snails in my ten-gallon tank (they stowed away on some plants YEARS ago and I haven't been able to get rid of them, so I stopped trying). Recently, in the last month or so, it appears that their shells have been turning white from the tip forward. They've also been burrowing less, lying around on the surface of the substrate during the day. I don't particularly love or hate the snails, but I don't like the way it looks. It looks like they're dying, and it's just plain depressing. At night though, they're still crawling all over the glass, so maybe they're okay. It's hard to tell.
I haven't really changed anything in the tank, except that around the same time, my Amazon Sword started dying, just the outside leaves. They turned brown and transparent from the base up and I cut them off, and the new leaves appear fine. Also, more algae has started growing on the leaves of the plants. I don't know if this is related or not.
Oh, and I just tested the water - ammonia and nitrites are 0, and the pH is about 6.6.
Anyone have any ideas as to what's going on here? I'd really appreciate it.