nassarius snail death?

[MysterRC]

So, noticed this morning I had a blue legged hermit crab sporting a new shell — a nassarius snail shell. A couple hours later, I found my skunk cleaner shrimp tugging away at another nassarius snail shell and he had a little pile of them in his corner. Looks like I had a bunch of them die… what would kill them all of a sudden? Meanwhile, I have recently stared finding baby nassarius snails crawling in various spots, so I dunno what is going on.

 

My tank is a 39G established tank that I moved. Water parameters have been hovering around SG 1.025, Ca 400, Ammonia 0, Nitrite 0, and Nitrate fairly high (20+) — been doing water changes to try to keep nitrates down. Everything else in the tank seems happy, just these mysterious deaths. If it was the nitrates I would expect other issues in the tank.

 

Thoughts?