I couldn't find the skeletal danio this morning, and this afternoon, it was dead.

Which is not exactly great, but isn't the end of the world. Things happen to fish once in a while, not much can be done about it.

Unfortunately, at least one and possibly two of the other danios are starting to display the same symptoms; not eating, hovering in place... no external symptoms, so if it's a disease, I don't know how to treat it. (Parasites...? but where would they have come from?) It could be a water quality issue, but my water quality has been getting better lately. (Having too many baby snails in the tank was causing a nitrate problem... but the levels seem to be okay currently.)

So far the other fish all seem fine...

But how can I even consider getting more fish if I can't keep the ones I've got alive?