Should I raise rent

[Xin Z.]

I have a rental property with slightly below market rent (around $25).  The tenant is one of my best tenants, and I hope they would stay longer.  Is it better to keep the rent as it is for one more year and raise the rent later, or should I raise the rent each year by a small amount like $25 to keep up with the market?

Please provide your comments and advice :)