[pogodzib]

Originally Posted by Pyrrhus

sounds like Velvet

http://edis.ifas.ufl.edu/pdffiles/VM/VM00400.pdf
amyloodinium treatments - Google Search

Lets not jump to conclusions. Velvet has external signs. Follow my previous recommendations and look for external signs of disease first. If you don't see anything, the best bet would be that there is something in your tank water.