Flicking is really annoying to diagnose because it can be caused by gill and/or skin irritation through parasites and/or water chemistry (not only including the usual nitrate/nitrite/ammonia suspects, but also metals or chlorine/chloramine in the water from insufficient conditionning).My suspicion is your water quality still.
Check your filter is running smoothly and that the water is aggitated so that plenty of oxygen is getting into the water and that your heater is working by checking your water temperature.
Huge water change and keep your eye on your water stats daily making sure you follow the test instructions carefully so there are no mistakes in your readings. You can easily bring your nitrates down by thorough gravel vacs (take everyhting out of the tank and really get down into the lower levels of the gravel) so that all poo and uneaten food is not left hanging around, cutting down on feeding and adding lots of real plants which will use some up.