Our ex-renter has left & not paid several months rent do i reduce the rental income $ or show it as an exspence?

Thank you for your question.

You need to put only the income you earned from him in 2015.

Please read this link to learn more about this form:

http://www.cra-arc.gc.ca/tx/bsnss/tpcs/rntl/bt/rprt/menu-eng.html

Have a good day.