While I very much concur with Magic Kiwi in regards to more info being welcome, if occam's razor is generally true, a tank with a canister filter, no export equipment (I assume the OP would have mentioned a skimmer in the original post if one was present), and persistently high nitrates is one of those things where I'd deal with the obvious first.
People who are successful with canisters long term are dealing with the nitrates produced. Whether they have set something up intentionally like a biopellet reactor, or they're lucky and have high innate de-nitrifying capacity in their sand and rocks and just don't know it, the nitrates are being made and they are being somehow consumed/exported/broken down.
Even cleaning the filter more often probably won't help, unless you're cleaning it every other day like you would with a filter sock. The food and poop bits that get trapped will be turned in to nitrate within a couple days of getting caught in the filter floss in a microbially active filter.
doing more water changes is definitely an option, but to deal with sky high nitrates you'd need to be doing massive water changes very frequently to make much of a dent. If hypothetically you have a tank with 100ppm nitrates, you'd need to do a 95% water change with nitrate free water to get the levels down to 'acceptable' in a single shot, or you'd need to do 13 20% water changes (2.5 times your total system volume) in a row, assuming no new nitrate was being added to the water column (which is most certainly is). More water changes will obviously help, but they're a horribly inefficient way of dealing with nitrates unless you're down with doing 65% water changes twice a week.
Dealing with nutrients in your system requires you to manage them on two ends - the production and the consumption/removal.
1.
Make sure your'e feeding them just as much as they can eat without much left after 2-5 minutes. If you're already doing that, it's not really fair to them to restrict their diet and make them go hungry as a means to dealing with nutrient problems when there are better, structural ways to deal with it.
2.
Find ways to limit the nitrification process. There's a great article in Advanced aquarist that deals with this whole thing that you can find here:
http://www.advancedaquarist.com/2003/8/chemistry but I'll quote the point that's relevant to this -
"Remove Existing Filters Designed To Facilitate The Nitrogen Cycle. Such filters do a fine job of processing ammonia to nitrite to nitrate, but do nothing with the nitrate. It is often non-intuitive to many aquarists, but removing such a filter altogether may actually help reduce nitrate. So slowly removing them and allowing more of the nitrogen processing to take place on and in the live rock and sand can be beneficial.
It is not that any less nitrate is produced when such a filter is removed, it is a question of what happens to the nitrate after it is produced.
When it is produced on the surface of media such as bioballs, it mixes into the entire water column, and then has to find its way, by diffusion, to the places where it may be reduced (inside of live rock and sand, for instance).
If it is produced on the surface of live rock or sand, then the local concentration of nitrate is higher there than in the first case above, and it is more likely to diffuse into the rock and sand to be reduced to N2."
If your tank doesn't have a sump, it might feel like you need the canister filter, but think about what that filter is actually doing and whether or not you need it. 95% of modern sumped tanks run without any bio-media at all (and unless you clean your canister filter floss every other day, it's biomedia) and none of them suffer an ammonia-pocalypse. If you've got even a moderate amount of rock and supplementary current you've probably got ample surface area for the necessary nitrification bacteria to do their job right in the display.
This also extends to trying to both encourage the consumption of waste by animals before the bacteria have a chance to get to it, and finding ways to remove the un-decomposed waste from a tank altogether. It's better for food that your fish miss to get eaten by pods and worms than decompose in a canister filter, and it's even better for it to be removed by a skimmer or filter sock that is changed daily or every other day. Do you have a skimmer?
3.
Add some element to your tank that is specifically dedicated to removing nitrate that does not require inefficient interventions with human muscle power (i.e., water changes). The advanced aquarist article has a few suggestions, one of which being a deep sand bed that a lot of people on here would never put in their tanks, but have you considered adding a small biopellet reactor to your system? You'd be far better served by replacing the canister with a small skimmer and pellet reactor than removing all your sand. Some people here won't get near biopellets either, but it's just one of many different systems that exist to process nitrate. There are also adsorbent resins, other kinds of reactors, fuges with exportable macro algae, etc.