I think we've been over-watering Plucky. When we went to check up on him a couple days ago, we realized he'd dropped a set of leaves, and after two days of not looking so well and the soil taking too long to dry up, we decided to transplant him into fresh soil. He ended up losing his whole stem with all his ferny leaves in the process, but I think it was for the best, because it would have taken a lot of energy to repair it. Growing a new stem is probably best.
Now we're gonna water him only when the soil starts looking more dry than moist. Apparently it's not supposed to be soggy, and it's been pretty soggy, so--oops. But never fear; he's still vibrantly green, so it looks like he's survived the flood!