No need of statements if the letter mentions 6 months average balance is True. Provided your average balance is very close to or higher than your present balance. If it is much lesser than your present balance, you have to conclusively explain the difference and prove the source of funds which is causing this difference.
In your case, if requirement is 18000 CAD, present balance is 18500 CAD and 6 months average balance is 17500 CAD, you don't need to provide balance statements of last 6 months because there is no significantly large difference that needs to be explained. Also note that, in this example, 6 month average balance is less than required funds and that is also fine.
Hope that clarifies. All the best!