It was not snails on the glass. I had specific numbers of hermits and only one snail, and it's huge. But no, it was nowhere near the glass. Yes, I've heard that glass clacking before. It's quite a different kind of sound though.
I'm not sure if I heard the sound tonight. It was about 30 seconds before the lights went out, which has its own kind of click. Actually, it's the same kind of click as the lights going out except, about 2/3's as loud. Sometimes quiter than that too. Like it's in a cave. But since it was so close to the lights out click, I'm suspicious.
Does this sound like the description of a click of a pistol shrimp "if" he is still alive?