For the record, I feel compelled to suggest some corrections to what others have posted here. I don't disagree with the intent, but want to make sure the content is correctly understood.
Canister filters don't "put out" or otherwise create nitrate. They are not, on their own, a source of nitrate. There is, however, a common notion in this hobby that they are inherently bad because they can lead to high nitrate levels.
Nitrate in our tanks occurs when other nitrogen-containing substances (mostly ammonia and nitrite) are broken down by nitrifying bacteria. These bacteria are sessile - that is, they find a nook or cranny somewhere in your tank and stay put, spawning generations of relatives. A small piece of sponge or filter floss can hold millions of bacteria - these types of materials are great at providing them with a good environment.
Hence, if you put a canister filter on your tank and fill it with flossy or porous media, then leave it alone, it'll soon colonize and start breaking down biological waste. One of the byproducts of this will be nitrate. This alone isn't a bad thing - nitrate is much less harmful than ammonia, for instance. However, in a sensitive reef tank, nitrate can cause all sorts of problems. Hence, most people would rather not let bacteria break down organic waste - instead, most modern hobbyists attempt to remove the waste completely from the system, through means such as skimming.
Of course, if you remove and replace or rinse off the media in a canister filter often enough, the bacteria will never gain a foothold. In this case, the media can perform as a mechanical filter, trapping and holding waste until you have a chance to remove it. But this would have to be a fairly regular job - as in several times a week - to prevent the media from becoming colonized.
All that said, I'll echo what others have suggested - don't try to blame the canister filter. But, make sure you are using it correctly and understand the implications. Evaluate your entire system, as the cause for your problems could likely be coming from elsewhere.