What caused this snail die-off?

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Expert Jeremy explains the possible reasons why a readers' tank snails have suddenly died...

Q) I have a 60 l nano tank. There are no fish, just live rock, a couple of coral frags, hermits and snails which I think came in with the rock. The base is coral sand and there are a couple of small pumps for flow. There's no skimmer as I don't intend to add fish. It's been set up for about three weeks. I recently moved the live rock about to make room for a new coral frag and about an hour later I noticed that the snails were dropping to the base of the tank and dying. The hermits seem fine, but I think I've lost every last snail. Any idea what might have caused this please? Did I disturb something in the substrate?

TIM FREEMAN, VIA EMAIL

A) Jeremy says: Test salinity, temperature, KH, ammonia, nitrite and nitrate. Dead spots can occur underneath rocks, usually manifested as black discolouration on the sand and a rotten egg smell. A trace of ammonia and nitrite may then have accompanied it, and that may have been enough to kill, but I don't think so.

The fact that they were dropping down and dying suggests a lack of oxygen, or a release of noxious gas from under the rock (both of which would have been helped by using a protein skimmer), or that the new coral frag has released a toxin into the water — is it a zoa or a paly frag, for example? Or has the frag literally just been made with a blob of epoxy putty? Putty can cause adverse reactions in small water volumes without skimmers, so in this case I’m leaning more towards either the epoxy or the frag itself causing the death of your snails.