Update
Thought I'd bring you all up to date on my progress. Travelinman's theory of swim bladder problems seemed sound to me, so I fed the danios a shelled pea one day, freeze-dried daphnia the next, and then fasted them for three days. This was in case there was any internal impaction.
The good news is that the danio who was originally showing the "lean" -- the one in the picture above -- is fine now. He's back on an even keel and is swimming around more and more.
The bad news is that the second pearl danio is still leaning, and one of the zebras seems to be having a little trouble, too. When the zebra tries to stop swimming for a second, his body immediately starts tilting upward at the tail end. The result is that he's constantly correcting himself.
Here is something else I've noticed in the fish that are affected by this. Maybe once or twice a day I notice that they will swim slowly up to a plant and then kind of "bounce" off it -- slap their body into it and swim away. They do this maybe half a dozen times and then stop. Seems like something akin to flashing, but I wonder why they do it so rarely.
An odd set of symptoms. Today I fed them Pepso Food in a continuing attempt to "clean them out." I'm heartened by the male pearl's apparent recovery, but I'd like to have this licked alogether -- these fish are supposed to go into a 30G community soon, but if they are still acting sick, I don't know if it's such a good idea. If you have any ideas, I'm eager to hear them!
Thanks in advance,
John