[Volker]

I think it could be some kind of infusoria. I guess it´s more a definition problem what kind of microorganisms we call infusoria.
I have here some paracemium which are good visible and some infusoriamix which is smaller in size and more like a fog.
I did some research and read that in tanks with no technic and no watercirculation an extreme infusoria development can happen. An explanation was that you have first a high bacteria development and than their predators follow(infusoria).
As solutions were mentioned to put a source for watermovement in the tank and
the moving clouds will become less(?even disappear?) or to put in daphnia or cyclops which will also decimate the microorganisms or their foodsource.
But it´s the question if the fishes will give them the time to do their job.:)