I am a California landlord.
My R/E attorney added a section in our residential Lease amendment that states:
"Maintenance of the toilets shall be the Tenants responsibility".

Is this correct and enforceable in California?

If the toilet wasn’t working/flushing, I would think it would depend on what the cause was. 

I use the California Association of Realtors residential lease, and it has a section under maintenance that reads: “Tenant shall be charged for repair of drain blockages or stoppages, unless caused by defective plumbing parts or tree roots invading sewer lines.”

So I wouldn’t see any issue with making the tenant responsible for calling a plumber and dealing with a clog they caused by flushing something that wasn’t meant to be flushed. However, I think you’d have a hard time pushing that same responsibility on the tenant if the cause turned out to be tree roots somewhere down the plumbing line (especially since working plumbing and a sewage disposal system both are required under the warranty of habitability in California).

Your attorney’s language doesn’t seem to differentiate between the two scenarios. I’ve actually had to deal with both scenarios (multiple times). If it were me, I’d suggest making it a little more clear who exactly is responsible for what before the situation presents itself. 

A toilet running or leaking is about 5x more likely than a clog in my experience.  Rotted floors and sky high water bills are in this future.  

The dreaded clogged toilet (at 2am, right? Never happened to me with 3 dozen units over 16 yrs)  is usually a simple fix anyway with a plunger or small snake. 

Having tenants 'maintain' their own toilets is one of the dumbest things I've ever heard suggested.  This lawyer has obviously never owned rentals. They will not tell you when it leaks.  Leaks lead to mold and rot. Maybe say they are financially responsible for clearing self-caused clogs.

Plus, requiring  your tenants self-repair can also bleed into gray areas of equitable interest.  Genius!

@Mark D. Just have your attorney amend this clause to say “tenant responsible for all maintenance”. Then when you get sued it won’t be surprising

I couldn't possibly agree more with @Steve Vaughan on this. The amount of stupid in this idea beggars belief.

Wait till you open up the mail box and pull out a 400$water bill then you’ll see the error of your ways . Don’t ask me how I know ! Or the phone call that the floor is somehow wet and you go over to realize the entire subfloor is rotted and the toilet is just laying on it rocking around , once again don’t ask me how I know ! Look you can put anything you want to void liability in that lease but it will be for naught when it costs you five times more than if you just did the maintenance yourself .

I will not recommend you to ask your tenants to self-repair, which will lead to a gray area of interest. A terrible clogged toilet is usually a simple repair. 

Agreed with @Steve Vaughan  

rule of thumb

clogs are tenant responsibility. When they flush condoms, diapers, paper towels.

Leaks are LL responsibility.