Yeah, that's a theory I considered heavily. There are urchins in the system, though no cucumbers. The problem with this is the lion--he was a lunker, and though he would chase small food items, he never ate them. The trigger, on the other hand, would go after anything and at least try it--so I keep thinking the trigger would be more likely to succumb to something like that than the lion. The mandarin and tang would certainly go after small food items--but I'd think the damsel and angel would as well. [shrug]
Sigh. I'm afraid this one might just be a stumper. All the remaining fish are still healthy and hungry in the hospital tank. The plan is to keep them out of the main tanks for at least a month--if it is a paraiste, it should die off without a host in that time; if it's a toxin, it should be removed by the carbon and skimmer by then.