Thanks everyone for so much help. I'm pretty sure I have "authentic"
otocinclus, my LFS seemed to be very familiar with them, and they do have
the solid black stripe down each side. They definitely are not CAE.

After I posted this I saw one going after the dorsal fin of a bala shark. I
watch my aquarium quite a bit, so two times is not "often". Usually (99.9%
of the time) they are munching away at green and brown algae.

I have a 55g tank with eight otos. Possibly one of them is not a true oto,
or as was said, a "bad" one.

I'll keep an eye. I think the scratching is getting better, I don't want to
medicate unless I have to. But like I said I'm keeping watch. Oh yes, just
by coincidence, I have a zuchinni coming in tonight, so maybe that will make
a difference.

thanks much,

bob