https://www.parosphromenus-project.org/en/forum/12-Methods/944-is-this-infusoria.html?start=6#954
Have a look if it looks like this – video on the second page. – (ps actually theres a better video on the first page of this topic)
I have that quite often, – and it often follows after I have attempted to feed fry, – by adding infusoria, – but it also sometimes appear without.
If I dismantle a tank, – which is old and ‘mature’ it may happen, – you know, if I dont take everything out, but trim the tank, it occurs. And I think its a sign that the tank is well… maybe too much food, too much organic waste – at least it shows the conditions for these infusoria is present, there is something there which they can live on. I have had tanks where it was really ‘thick’ with these almost clouds of tiny living things, – and you can see them kind of move in ‘clouds’.
It does not seem to bother the fish, and on the positive side its probably very good fry food – on the negative side it may say that the tank need a ‘clean-up’
Unless … well, – did you get it in with the rainwater do you think ? … or did it get stirred up from adding the new water ?