I use the crushed cuttlebone sandy type perches for my birds but honestly every time I've tried something like it for my rats,they've just chewed it to pieces.
I have found that bricks in the cage placed where they must step on it
(under water bottle or used as steps from room to room)works better.
My cage is actually 2 Martin's R680s connected with ferret tubing so that's where my bricks are, at the door ways where the tubing connects them, they have to walk on the bricks all the time.
I hate clipping rat nails because they're so teensy, I have a hard time seeing them, I like my bricks so much!
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PHRatz