Two issues with my tenant...

[Luis Rodriguez]

My wife & I have a single mom with three kids upstairs.  The mom is over all the time. We've even seen her in her PJ's multiple times. My wife has an issue with her being over all the time since it looks like she lives upstairs. I don't really care, they're quiet and pay rent on time.

Now, our Jan-March water bill is about 50% more compared to any bill in the last three years. We thought it was a one time thing, but we're on pace for the same usage on the next bill. We bought the house in Oct. and it's just the two of us in the first floor unit. We know the previous owner and tenants were 8 total people. 

If there are no leaks, how do I proceed ? im in NYC, where the laws really favor the tenant.