Take plumbing repair as example for sake of fairness

Let's take plumbing repair as example. If the faucet is leaking, you as landlord is responsible to repair or replace it. You may argue that the tenants abuse it. Well it's hard to defend yourself that this is not normal wear out. The only thing that you need to be cautious is to consider # of people occupying your property. The more peoople, the high wear out rate.

On the other hand, the drain problem is definitely caused by the tenants if it's not pre-existing problem prior to move-in. The tenants can't argue that this is not what they caused.

Compromised solution. If you tenants can barely come up with the rent payment, they can afford the repair cost. You may ask them to share the cost more or less. Or you can say:  " I take care of the cost of unclogging the drain this time. Next time you pay for it." that will make them more conscious about the financially responsibility for their abuse of the facility.

You do want to make the lease fair to the tenants by asking them to review the lease caerfully before signing. The terms can be negotiated further to reach mutual agreement.

Give it a try and good luck.