Losing a snail everyday

[VioletThePurple]

Every day for the past three days I have been losing a mystery snail. My nitrates were very high (40 ppm) so I've been doing water changes every day to get them down. Now it's at 5ppm and still losing snails. The only symptom they have before dying is that they turn greyish. Still active, still eating, no unusual spots or tumors, and the shells are perfectly intact (but empty and I end up finding the body on the substrate floor). My PH is 6.0, no nitrites. I recently added a couple new platies that show no signs of illness. I did lose a fish when my nitrates were high, but I haven't lost any more fish since getting my nitrates down. But I've still been losing one snail per day. The tank isn't new; it is at least a year old. Unless my platies or cories are eating the snails, there are no other fish in the tank. I have had my snails for three months prior to this with no issues.

 

[Abaddon]

How long have you had them? Is it possible that they might just be dying of old age (if you've had them for more than 1-2 years then it's not inconceivable)?
The only potential issue here other than the nitrates is the pH, which is very low for snails (they prefer neutral to to slightly higher levels). Have you noticed any signs of shell damage/erosion? If so, that could be the cause since it's the most obvious thing that's off in this scenario.

If you're finding bodies, it means the fish are not eating them These snails have a "trapdoor" so are usually able to hide from predators. Your fish would have to be pecking at them relentlessly to cause them to actually die imho; I don't think this is the cause tbh.
And I don't think 40ppm nitrates is enough to kill them off unless the levels were this high for a very long time, or the snails were already weak/old.

It's a bit of a reach, but poisoning is another potential reason (esp. copper) - have you used any new meds or fertilizers recently?