A Canister Filter (or any mechanical filtration) is "TOO" good at what it does. Instead of allowing detritus to be broken down here, there, all over the place it gets caught and broken down quickly into Ammonia, then Nitrite, then Nitrate. The problem is it can't process Nitrates so they end up building up in your system. If they aren't maintained VERY often they become a "Nitrate Factory". It only takes a busy weekend (or two) to catch up with you and cause serious Nitrate problems.