Canister filters produce a lot of nitrates or are often referred to as "nitrate factories" if they are not cleaned very frequently. Same goes for bioballs and wet/dry filters. They trap lots of waste that gets broken down by bacteria. They are a perfect habitat for bacteria that break down ammonia to nitrite and then nitrate, problem is they do it so prolifically that nitrates are hard to keep in check. With saltwater tanks nitrates need to be kept much lower than in freshwater systems because fish are much more sensitive and because the holding capacity of water is significantly less because of all the salt.
In saltwater tanks, live rock provides the surface area for bacteria and deep inside porous live rock are anaerobic areas that break down nitrates. Canister filters don't have the anaerobic areas that live rock does. So if the canister filter isn't cleaned frequently(like at minimum once a week, preferably more) it will make keeping a saltwater tank difficult. You will always be fighting nitrates and phosphates.
Doing water changes, cleaning the canister, having a quality overrated skimmer, good water circulation with a good amount of live rock, feeding lightly, and stocking lightly can all help you be successful with the equipment you have. It isn't impossible, just more difficult than with a sump.
