12-16-2012, 01:42 PM
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I think you answered this for yourself, if this came out of your filter it needs changing badly.
Sorry can't say if it is bloom or bacterial matter.
If this had happened to me I would have completely disenfected the tank, new gravel and fully cycled before putting any fish in the tank.
To be safe,,,,Get your Danios into a hospital tank, clean, recycle your tank, renew your filter medium,,,,,,
Let me know how you get on.