Maybe you need to put a filter-replacement clause in your lease agreement. The tenant may have thought it was your responsibility.
It’s not in the lease but I had once spoken to the guy about it, so he knew. He said he hadn’t done it because he didn’t know where the filter went. Apparently he did not appreciate the seriousness of it.
Though you’re right that it should have been in the lease, because had it been in the lease I would probably have been more forceful when I did speak to him about it. I sort of inherited the lease from the previous owner, in that I pretty much copied the lease that the previous tenant had with them. (The previous owner was the one who built the entire subdivision - they held back a bunch of units to rent out - so I figured they knew what they were doing.)