So, I have two concerns:
1) If I change the water weekly, will the oxygen levels be okay without an air pump?
2) Luna has a habit of propping herself up in corners so that her side is sticking partway out of the water. In relation to the oxygen question, I was afraid she's doing this because she can't breathe, but a friend explained this to me as 'crayfish just like corners' and recommended putting bubble wrap in to round them out. Do you have any other takes on why she's doing this? Is it harmful? How would you recommend stopping it, if necessary?

(Yes, I know she's missing a claw. It wasn't in her mailing bag, so I can only assume it happened when she was younger. I suppose that's a bright side?)
Thanks!