Look closely Bob... does it look EXACTLY like that picture? Do you remember the eye placement... probably not from the quick look you got. Eyes towards the front like your pic indicates a parasitic isopod... could be a Cirolanid. That's not good news. But it also could be an innocent sphaeromatid like these:
I have a pic here somewehere that shows a comparison of the 2... I'll find it.
So there was only one of these guys? And it was swimming? Keep a close eye on your fish....
