rent dispute over $20 increase-what to do?

[Marci Stein]

Tenants have been renting for 4 years now ALWAYS on time...They are perfect tenants. However, i noticed the lease states rent is $ 895.00 a month, NOT $875.00 

I just noticed this after 3 years (I know). I asked them to start paying the agreed amount on lease and they said they have been paying the lesser amount for 3 years and i accepted it so it should continue till end of lease (another 16 months). 

Am I unfair to ask more? I don't wan to rock the boat as they are good tenants but they did agree to the higher amt when signed lease . 

What would you do??