Tenant is violating city code - what do I do?

[Ryan Cooley]

We received a letter in the mail from the City in which we own a rental property. 

It's a letter of warning that they've received multiple complaints about the tenants' dog running around the neighborhood and chasing people. 

This is a city code violation. 

How should I handle this? If the tenant doesn't address the issue who is in trouble for this?