Landlord expecting correct payment for water charge

I have a large custom 2 family house in a boro of NYC. I reside in one apartment with my son.
I rent the other large apartment to a couple for a few years now with a Lease.
The 2 year Lease was renewed last year for another 2 years.
I have one large water heater and one water meter. I got talked down in rent when they moved in they seemed nice.
 I did request an estimated water payment of $25.00 and in the Lease. Lease states water charge can change at anytime deemed necessary due to Water Board increase or usage.
Note: there are 2 tenants in the rental apt, and 2 people in my apt. also.
What I did is take the 4 previose water bills, which I have had 2 people in the rental apt. also, add them together and devided it in half. then devided it by 12 to get estimate of water use for each apartment. I do not want to cheat my tenants. In the Lease there is a clause stating they can’t bring another person to live in there apartment.
About 6 weeks ago they had a baby. At this point I requested a $12.50 estimated increase for an additional person water use. Yes it is a baby, but they have a washer and dryer in the basement. There has been lots of small washes of baby clothes, I am not sure if they adjust water levels or not for the wash. I do not think this is important to them. My tenants claim they do not have to pay for water for a baby, it doesn’t use much water. Well I disagree with that, The baby is here about 6 weeks now and I received a water bill that was for the last 3 months and I compaired it to same months but for previouse year and found the new bill is $45.00 more than the last years bill for same period regarding weather conditions. On top of this there has also been a 7% water increase that I have no control of. My Tenant has refused to pay the additional water that I am requesting.
I believe this baby is a person and I should have the right to add additional water charge and due to water estimated use.  
If a baby doesn’t count as a person that uses water equally, then just when does a person count as someone that uses water. Tenant said doesn’t use same amount of water as an adult. Didn’t pay the 7% water increase also.
Kind of acused me of ripping them off and they are now banging the door when the baby is not sleeping.
They painted the apartment a dark color even though it was in the lease they can’t paint until I give them letter approving of light color they picked. She works in the house even though the Lease states can not use apartment for a business. When they received an increase after 2 years I was to demand security to match the rent as per lease and let it go to be nice.
Does anyone know what my rights are in NYC area regarding this with a 2 family house.
Note this is stupid due to I am willing to make adjustments either way to adjust there correct water charge. I believe I have the right to charge water for a third person. Lease doesn’t state anything about a 3 person or baby but it says I have the right to adjust water charge at anytime deemed necessary.
I have always been nice, maybe to nice. This house does not bring in a profit at all. I keep adding money back in and also whatever I can afford to fix everything I can all over the house including the tenants apartment. There is no government agency that could help me

I would appreciate your response.

Thanks so much.

Just would like to say, it is too bad that tenants do not understand a good landlord that tries to takes care of things, and always looking to fix whatever needs to be fixed as much as money allows and pays high insurance and taxes does not make anything at all. I also work to do many things myself so I would have money to do the things I can’t do. I do not understand why I get treated like I am getting rich. I work so hard in this house also. It would be easier to be a tenant than what I do. And I am getting older getting hard, but still trying.
Thanks.