Figure out why you have nitrate. It is as basic is nutrients in and nutrients out. The ways in are mainly water and food, but in a way can be filter media as well.
The water you use for water changes and topping off can contain nitrate and be a significant source in many tanks. Use RO
only for both water changes and top off.
Food is the other main source. Every bit of food you put in the tank will eventually be broken down into nitrate and phosphate. By feeding what is needed, and little to no more, you can keep this input to a minimum.
Filter media is an often overlooked source. You say you have a canister, nitrate production is probably the biggest reason why canisters are not used on most saltwater setups. ANY mechanical filtration traps debris. That debris will breakdown and can produce a massive amount of nitrate and phosphate. It is vital that you clean any mechanical media at least once a month to prevent what is usually called a nitrate factory.
Methods of removing nitrate are water changes, denitrification (usually through live rock), and special filters.
Water changes are the ideal method. They remove nitrate and other bad things while also adding all the good stuff back into the system. A frequent water change schedule will prevent most of the problems encountered in the hobby. I suggest at least 10% weekly on a reef tank.
Natural denitrification via live rock is one of the most important benefits of live rock. You must have a proper amount of live rock to achieve this though. Most people suggest 1.5-2 pounds per gallon. I have 3 per gallon in my reef. You also need flow all around the live rock for it to get the flow it needs to function properly. This includes a powerhead pointed behind the rockwork, not just pointed at the front.
The last and usually least efficient way of removing nitrate is with special filters or filter medias. In most cases these do relatively little and would only be a good option if you have optimized the other methods and still need to get that 5-10ppm down to 0 or 5ppm.