How does "oldest debts get paid first" clause work in lease

[Bienes Raices]

I would like to add an "oldest debts will be paid first from the rent" clause to my lease, but I'm a bit confused about how this is applied in the real world.

Let's say the rent is $1000/mo.

They pay the full $1000 for February rent, but they never paid, say, a rent late fee or a water bill (that was in my name but they were required to pay me each month) that was due in January, for $30.

So with the rule applied, they've only met $970 of their $1000 obligation for February rent. However, if I accept the $1000, technically I'm accepting a partial payment of rent, which is a bad idea. So would the idea be to just refuse the $1000 payment and post the 3 day notice at that point, or are these "old debts" just carried on and removed from the security deposit at the point when they move out? Thanks.

[Jon Holdman]

Why add a clause to your lease that's hard to understand? I have this clause in mine:

This means in your example, if I accepted the $1000, $30 would go to the other items, leaving $30 of the rent unpaid. I also have this clause:

I would not let the tenant repeatedly partially pay, as in your example, then take the rest out of the security deposit. The first clause above is specifically to give me the ability to evict if they don't pay EVERYTHING they owe.