Okay guys it's getting bad. I finally fed them again today because everything's been normal for the past couple of days. They were hungry and happy, but then they just started flashing really badly. Especially my biggest fish, who I really can't afford to lose. She keeps swimming under the water flow and flashing. Another is following her lead. It's far more repetitive than it's been. Why is it starting up again? The flashing points to parasites, but the timing is weird...only at night, more violent after feeding?
Currently doing a water change, despite some differing opinions, because I honestly don't know what else to do and it's helped in the past. As soon as I turned the pumps to spill water outside the pond, all the fish calmed down and stopped flashing.