I have a 75g tank setup for livebearers - mollies, swordtails and platys. I have about 20 of them and some of their fry. The tank is properly cycled and been running for 6 months now with two external filters with bio media and two sponge filters. They seem to be thriving and breeding but the one thing that bugs me is that they flash against plant leaves and gravel from time to time. Sometimes they do it over and over. No visible ich even with all this flashing since starting the tank. They do rarely get some small fungus, but disappear after a few days. No fish has died or sick from the flashing, but obviously they're irritated by something in the water and I can't figure it out. Any suggestion to what I can check or do?
Water Parameters: Temp: 80F, PH: 7.8, GH: 300ppm, KH: 80ppm, Nitrate: <10mg/l, ammonia and nitrite 0. 1 tbs salt added for 5 gallon of water.