yes, you absolutely have to dose N03! IMO your corals consume it fast. I test my n03 every few days, actually if your no3 is at or under 5 i think tanks will not do well. once you get your no3 up you have to be careful to keep it up because once it its up and it drops it can crash your tank... also the tank will take a lot of n03 to become saturated with it. at 1st you will dose and dose it and it will not come up. this is because the starving corals are consuming it very fast, but once the tank is saturated with it you can dose less to maintain the value.
for example, in my 540-gallon system. when the no3 drops below 5 it will take me like 400ML a day of neontiro for 5 days to bring the n03 back to 16-32 ppm. but once the no3 is up there i can maintain it at 125ml per day. it takes some practice to keep proper no3 but this is IMO the key to good coloration and polyp extension.
Most people get frustrated with adding the no3 for the 1st few days because they can't seem to the value up and then testing it is a pain so they start feeding more. feeding more will just dirty up the tank and cause other issues. I use a controlled feeding method from American Reef HPD which actually brings the no3 down and po4 down to close to 0.. then i control it by dosing it up....