Thank you very much to you and Diana !
That's very interesting.
The mecanism seems to be very similar to some human mycosis, that happen because our usual bacterian flora becomes unbalanced. This can happen for example when saprophytic bacteria are killed by disinfectants... then pathogenic organisms as Candida can develop and we get mycosis.
Ultra-cleaned environments are not healthy, even if it can be surprising !!
It's very useful too, to know that UV filters can kill these bacteria, and that they mainly stay in the cleaned water more than in filters where other bacteria keep them low. So when we can, we'd better use UV filters instead of general disinfectants for our tanks, sometimes : this could prevent the selection and development of mycobacteria. Or maybe filter on UV after a treatment, to put all bacteria to the same number again ?