I have a 40 litre tank and lately I saw that most, if not all, of the guppies had clamped fins and rubbing against the surface. I suspected ICh but there were no signs of white spots. The next day I had three or four dead. I checked the water parameters, everything was according to a mature tank 0 - Ammonia and Nitrites and a bit over 20 for nitrates. I did a water change but to no avail. More deaths followed after some hours. I am lost and at this stage do not know waht to do