Lease ends, tenant moves out, and then you learn that they broke the No Pet Rule. What do you do?

[Charlie B.]

Would you keep a "Pet Fee" out of the security deposit?  In this instance I am holding 1.5 months deposit ($1,725).  Otherwise the tenants were perfectly pleasant.  So what do you think is fair in this instance? We're replacing the carpet- not necessarily due to the cat but it is a factor.  Thanks!