They were alright and had there uses to help polish and clean the water. I was using Boyd's chemi pure with resins and their carbon when I used my canister filter. I would only use it for up to 3 days. I really have not used it in a couple of years now. I just felt it was not the way to go, unless you had some real trouble going on and had to get control of it right away.
I have had more faith in the poly filter personally. It helps with several things at one time. Nitrates, Phospahtes, copper, and especially found the poly filter great to help pull out toxins from the red flatworms when they die.
Still looking for the cause of your nitrates is what is needed, then a control method to handle them.
Thomas