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The usual recommendation is to remove all the fish, treat them, etc. And leave your tank fallow for the recommended time periods. Especially since one fish is showing symptoms, and since these diseases can rapidly take over. Not every fish will die 'at the same time'. As to whether you should treat asymptomatic fish with formaldehyde - I'll leave that to the experts (I might not) - but I would use copper as directed, making sure the fish you're putting into copper will tolerate it. Hopefully the others will weigh back in.
The LFS said all the white looking stuff On my rocks is brook to. Is this accurate? I thought it was algae bleaching out because my light intensity went up for the first time in 3 months.