The three or four of the smallest tetras always have a ragged fin edge or two; usually they heal and grow back fine. In fact, only one serpae in the tank has fins in perfect condition- the second-largest of my fish, I bet it is the dominant one and simply doesn't get nipped by anyone else. I saw one day that one of the smaller ones had a white blob on edge of dorsal fin where it's uneven. This might be a bit of fungus infection starting. At the same time, noticed there's a slight film on water surface, I wondered if I'd been overdoing it feeding them live foods. Checked the water quality, nitrates were normal at 20ppm, but I went ahead and did a small partial wc anyway. Since I wanted to remove some of that surface film, I was scooping water out with a cup, making a strong backflow to gather some leaf debris that was floating, too.