I would tend to agree with Bob. Could this be a first...
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If unable to diagnose the problem, treatment is futile and the stress of moving could be detrimental as mentioned.
Get back to the "basics" and try and find a possible issue with water quality ? Consistancy with temp. etc ? Any changes in the past since this began or has it been since the beginning ? This is when a log book is handy but I too fail to keep one.
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Compatability issue ? External stress ? (Hands in the tank too often).
My initial response is to replace the GAC and do it often to rid the system of a possible contaminant. Ensure all filtration and powerheads are functioning properly.
All the best ! Keep us posted.