Poison will only kill a subset of the mice which get inside. It's treating the symptoms, and not the cause. Plus, you wind up with dead mice behind your walls slowly mummifying away.
Figure out what's attracting the mice into your units in the first place. Eliminate as many points of entry as possible with caulk and spray foam (mice can fit into any crack big enough to poke a standard pencil through). Cut brush back around your foundation and secure anything that can become mouse food (trash cans, pet food, recycling).
This. We had rats and mice in the rental I live in (an 1880s Victorian that had the sub-floor exposed while the landlord was working on the front steps... ack). What worked was eliminating points of entry (the exterminator we had used steel wool to stuff some holes around the foundation instead of caulk/form, since rats and mice won't usually chew on steel wool to re-open the hole). Look around your foundation, and your roof as well; if you see little smudge marks you're definitely seeing an entry point. If you have roof access point, you may need to check about cutting tree branches, blocking wires, etc in addition to eliminating entry points.
Once we had that dealt with, trapping the mice and such was easy. We used regular traps, not poison, as we didn't want to deal with the mummified mice issue + we had a cat (plus it's friendlier to neighborhood cats, owls, and other creatures that may try to finish off a poisoned mouse). We haven't had mice in months.
It sounds like your foundation may be a bit trickier than ours. However, persistence will pay off -- even just reducing the number of entry points will likely make it easier on you in the long run, and I suspect your renters will appreciate your hard effort, too.