Apartment Unfair Water/Sewer Billing from the City

[Lucas Miles]

Purchased 2 small multifamily buildings in a very small rural town in Southern MN recently. The city is billing us $90 in base fees for every apartment (then in additional also a water usage rate), so 20 unit building we are getting billed $1800 + usage. There is also an assisted living retirement building in the town, the city is billing them only 1 $90 base rate for the entire building (not per apartment). Buildings are setup identical, 1 water meter in the building. We use 50k gallons of water is costs us ~$2000, they use 50k gallons of water they pay ~$200, obviously drastically unfair, (and extremely expensive for us) These are the only apartment type residences in the town. 

We have approached the city asking them to change this billing to make it fair/affordable, they declined saying the assisted living building is "grandfathered" in to pay the 1 base fee. 

Looking for any guidance for anyone that has dealt with a similar situation before, or if there are any legal precedence we can take. Thanks in advance!