Okay, it's not going to be an infection in all of those fish so that can be ruled out. My guess is that it's the TEMPERATURE of the water. That is the only importnant thing that you didn't post. If it's anything less than 84 that may very well explain the skittishness of the fish. It happens to mine sometimes when I do large water changes. The submersible heater comes out of the water since I change so much and it turns off and doesn't reset itself. I would do a 50% water change in your tank right now to get the nitrate level down to 10ppm. It certainly isn't helping that the level is that high. Ideally you should have the tank at 86 degrees, IMO. You will notice very different behavior even from a 4 degree change.