Nibblegar

It haunts the reed-shadows of Woodlow's slow water, hanging motionless nose-upstream, then striking sideways at anything that fits in its mouth and several things that do not. Fights among a shoal are constant and mostly bloodless, ritual nips that sort out who eats first. It grows alarmingly fast on a diet of whatever it can catch, and a pond that had a dozen Nibblegar in spring will, by autumn, have three much larger ones and a great many bad memories.