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These just started showing up any ideas? they aren't actually complete circles they kind of look like a C thats collapsed on itself. As far as I know they don't move.
I'm not sure what causes them either. I had a bit of an outbreak a couple of weeks ago. I scrubbed them off. I got my new protein skimmer a few days later and haven't had a problem since. I don't know if that is a coincidence or the fix.