Other chemicals, cleaning filter baskets, backwash, complete filter tear downs are regular maintenance items.  

By the time you buy a snake or plunger or chemicals (eg draino) and the back and forth, paying a plumber or handyman to do it is better. 

We charge clients for a replenishment of hot tub chemicals - usually once per year, as well as $30 per month for pest control. 

For both options the treatment will be using the same method but different product / chemical being used.

Because of Code differences, you will most likely also be looking at tearing out the basement ceiling to install a fire retardent barrier between the units, as well as other code enforcements.

I was afraid that a pro would either use harmful chemicals that would hurt the pets or that they'd simply be unable to seal the building to keep mice out in the future.

I didn't major in finance, rather Chemical Engineering, and if time were rewound I would do it again. 

This implies if the LL takes one month to place a tenant and a couple hundred dollars doing the always tenant turnover items (for example we always change lock cylinders, shampoo carpets, chemically treat the drains, deoderize/disinfect including use of ozoner). 

You will own a property that has these risks and 100 more:car falls on leg of customer and he is young and incapacitated and sues you tootenant pours poisonous degreaser into public plumbing an it leaks 200 feet to neighbor properties, they sue yourobbery happens and child is trapped in the bay and survives, the parents sue youteen comes in with their helper dog who bites clients, they sue youfireman falls through the roof in a fire (chemical fire) and is disabled because the roof wasn't to code...Although you are only the landlord you have duties to health and safety of the employees, clients who walk in, and the community.Look up: Ng to the Ghost Ship landlord in East Oakland

@Paul VailSomething else to be concerned about Meth houses is; where the occupants were dumping their waste chemicals.