Thank you both for the replies! I hadn't thought of a deficiency, but this happened over the course of maybe 5 days (going from what, to my eyes, looked like a healthy leaf to having holes punched in it), so is that too fast to be the effects of a just a deficiency? Many of these plants are brand new to me, so it's hard to say what they might have been deficient in before. My tank is organic pond soil capped with gravel, but perhaps I should be dosing extra in the water column?
So do you think it's more likely that the leaf was somehow ailing already, even though maybe not visibly, and the snails attacked it because of that? There was never a ton of algae on the leaves, but there were some diatoms that the snails cleaned off in the past (of the plants that aren't new to me). I really want to have snails in my tank, because they can create such a balanced ecosystem, but I'm so afraid of them doing damage. Among the plants in the tank are about $60-worth of my dream red sword plants, and I'm especially panicked about those just becoming an expensive snack.
We weep for a bird's cry, but not for a fish's blood. Blessed are those with a voice.
- Mamoru Oshii