how do you manage plumbing clean out costs?

[Lilith M Haas]

I have new tenants who moved into my SF rental in Riverside County, CA October 8th. In less than 2 weeks they managed to back up the plumbing on a Saturday and I have a bill to snake out & clear the line for almost $500. I had a new main line put in 3 years ago and the bill states that the clog was near the bathrooms and the cleanout to the mainline to the street did not remove any debris or roots (there is a city tree in front that can't be removed). I have not had a chance to speak to my property manager about this yet. He seemed to imply we can tell if it is the tenants fault, but I am stuck with the bill as of now. I have had that problem with tenants before and wonder if anyone can share how they minimize these costs?