What can I legally fine tenants for?

[Carol Venolia]

I know, that Subject question sounds kind of nasty, but I'm really a pretty nice landlord. However, I have some ongoing issues with some tenants, and I'm wondering if I can add a fine to their lease. (I'm in California.)

I know it's legal to charge a fine for late rent, but can I charge a fine for other lease infractions that I don't deem serious enough (yet) to kick the tenants out? In particular, given that my Pet Agreement states that the tenant will quickly and safely dispose of all pet waste, may I legally add a $25 fine for each time I catch them leaving pet waste on the ground?

(This is, by the way, a multifamily property with no private yards, so any pet waste becomes everyone's problem--and these folks really like to write their own rules on the matter.)

Thanks for any enlightenment you can offer--and happy holiday!