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Between my turtles scutes are grey seams. Its pretty bad now. I dont use any shell conditioner. I haven't tried to get it off yet eather. But Its creepy. Its bright gray. Like a grey sharpie. And his tank mate has it too.

If you can post a picture.
It sounds like growth lines to me. So long as they aren't too big, it's perfectly normal.

You can see it in my avatar if you look closely.

Your turtle looks very healthy to me. If you are worried, do a Google search on shell decay, or shell rot. That should show you what to look for.

Sounds like its just normal growth, like meg said, as long as they aren't too big your fine.

They don't come off. Don't use the shell conditioners either. They only lead to problems.