Would marks from a #2 pencil (in the US) be safe in a tank?

[kdx7214]

My wife has been making filter socks for my tank. I've offered to do it, but she loves sewing. She wants to know if she uses a #2 pencil to mark lines for cutting if that would be a problem.
I know that pencil "lead" is just graphite and clay, but I'm not certain of the components in either.

 

[The_Paradox]

Will be fine. If you have any doubts just wash it first.