After what I thought was a natural lifetime, my two nerite snails died. I went to the LFS, purchased a few more fish and two nerites. Within 36 hours they dropped off the wall and were quite dead. NO evidence of a softening, decaying shell. Meanwhile the new fish along with the existing fish & shrimp were just fine. Thinking I had received a bad batch, I went back and got two more. Same thing happened. The aquarium water is very soft and the other readings are fine. I have notched up the nitrate values a bit to help the plants. Are snails sensitive to nitrate?