Is this legal, and what can be done?

We just found out that our new apartment complex in Sacramento, CA, requires tenants to pay a SHARED water bill. Basically, this is a studio apartment, and even if we conserve water, we will still be paying for others who use more. Their explanation for this issue is that the buildings are old, and they can't change it so that each unit is measured by itself. When I asked why they can't just pro-rate the usage per unit, say making it so that studio apartments pay less than a 3 bedroom housing an entire family, they hung up on me. Is there anything I can do? This bill issue has me worried that we won't make the payments every month without me getting a third job. Thanks in advance!