Approach on a rent increase

[Christopher DeAraujo]

First timer here! take it easy on me.

I have 4 tenants paying well below fair market rent.  Current rents are 1300/1100/1100/1150.  Fair market is 1800.  My state laws do not put a limit on increase amount or frequency of increases.  How should i approach this with respect to losing these folks? average tenant length of stay is 7-8 years.  

I know this is my own fault.  Hoping to recoup some losses without having turnover and the costs associated with that

thanks!