Ich?

[marie]

A quarantine tank can be as simple as a rubbermaid container with a heater and a powerhead.
If you have an old hang on back filter hanging around you can keep the sponges for it in the sump of your main tank and when you need to set up a hospital tank, the sponge is already seeded with necessary bacteria to process ammonia (Just don't run carbon in the filter if you need to medicate)

If what you have is marine velvet then the only treatment that worked for me is copper.

Also google brookynella, it could very well be that