Looks plump, and keeps "darting" about with extreme amounts of what i assume is Poop. Is this normal to poop so much and make this funny swimmimg fashion. They aren't being over fed and the waters fine.
ZD are very active. The plumpness could be a female, however you say extramly stringy poo. That is a sign of overfeeeding or of an intestinal problem. try not feeding for 2-3days and see if there is any reduction in the poo.