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I have had my turtle for about 12 years and all of a sudden his shell is white in the left corner of his shell and i was wondering what could this be?
He has had this for about 3 months and it has started to get smaller.

What kind of set up does your turtle have? Lighting, size, temps, etc...

Do you have any pictures that you could post of the spot?

Does it feel squishy? Does it have a bad smell?

If it's hard, does it rub off with vinegar?

You said it's getting smaller...Any chance he's shedding in that spot?