Can I raise the rents for some units but not all in my fourplex?

[Randy Babineau]

I currently own a fourplex and have raised the rents for all units over the last two years of owning the property, but the rents are still below average for the area. If I decide to raise the rents again, are there any laws that I have to raise the rents for all units at the same rate? All units are approximately in the same condition and size, also all of the tenants are longer term tenants. One of my tenants is also a plumber and does a lot of work for me at a discounted rate, so I dont want to raise his rent as high as the others. I do have one tenant that is on Section 8. So the question is, can I raise one or two of the tenants rents, without raising the others? Is that discriminatory to the other tenants? Thank you in advance!