Again, what's the reason for these clogs? If the tenants have flushed something down the toilet that's more than the usual "bodily fluids" (i.e., sanitary napkins, childrens toys, cooking grease) the cost of unclogging is on them. If the toilet clog is due to, say, tree roots growing into the sewer lines, the cost of dealing with this is on the landlord.
Gail