The reference to tench made me laugh and I'll tell you why! About 10 years ago I saw this large fish struggling in a small tank in a local store, so decided to buy it. I was told it was a tench and promptly called it Terry! It has always been a bit of a loner but doesn't seem to mind the other goldfish. Anyway last year I visted a garden centre and asked what the large fish were and was told they were golden orfe. You guessed it, mine is an orfe, so I should have called it Oliver!
Sorry I can't help about the tench question.