One of your problems is the aquarium gravel. Aquarium gravel traps detritus and even when you siphon it regularly a whole lot will still get left in it.
The other may be the canister if you're not cleaning it out regularly.
Yes canisters CAN be nitrate factories but that requires an explanation.
Canisters aren't in and of themselves nitrate factories. You can't just stick a canister filter on your tank and it starts producing nitrates or something. It is the way that canisters are built that will potentially cause you nitrate problems eventually.
Detritus will build up on the filter pads and you will start growing a large colony of bacteria to deal with all this waste. Aerobic bacteria puts off nitrates as the end product, so if you are not cleaning out your canister frequently and replacing the polishing pads then you are only adding to your nitrate problem.
Also, if you are running biological media, like filter scrubbers, ceramic rings, etc. you are adding to your nitrate problems because these are meant to grow aerobic bacteria.
In a saltwater tank your nitrates are kept in check by a few components (Besides water changes):
A protein skimmer - A good protein skimmer will eliminate a certain amount of proteins from your tank BEFORE they turn to ammonia, which is then turned to nitrites by your bacteria, and the end product being nitrates.
Liverock - Deep in your liverock is a certain amount of anaerobic bacteria (don't mix this up with aerobic bacteria, the type that eats ammonia and nitrites), which disposes of nitrates. (The nitrates don't disappear but that's not important right now).
Deep sand bed - If you have a deep enough sandbed there will be anerobic bacteria growing within just like deep in your liverock to eliminate nitrates.
Algae/plants - The algae on the surfaces of your aquarium and any plants you put in, assuming they are healthy and doing well will absorb nitrates.
So, in your aquarium it sounds like you have a large amount of liverock and this is good, BUT you have aquarium gravel which traps waste BADLY unless you siphon it constantly and the aquarium gravel also doesn't grow anaerobic bacteria like a deep sand bed would. You have a potentially ineffecient skimmer (prisms don't have a good reputation but I haven't tried them so I'm going by reviews and whatnot), and unless you've been cleaning out your canister and maintaining it properly it basically has waste rotting in it producing ammonia which eventually ends up as nitrates.
An understanding of all these components should allow you to eventually get your nitrate problems completely under control.
