This confused the tar out if me for a long time. I was a saltwater guy for years before I ever tried freshwater so.. lol Plants actually need nitrates to grow well. 10-20ppm is much better for plants than 5ppm. It actually a fert.
Now as for ferts, once a week dosing is okay without co2 but once you have co2 going you'll want to dose more often, preferably every day. Plants will outgrow algae as long as they have the components they need to build new plant matter. If anything is in short supply the plant will stop growing and algae will take hold. With plants you want to look at your tank in terms of limiting factors or what may be in short supply for your plants. You have bright lights so light isn't limited. You have pressurized co2 so that should be fine as long as your drop checker is green from the time your lights turn on. You'll want nutrients in the substrate so root nutrients aren't limited, and fertilizers in the water so column feeding plants aren't limited.
There are more knowledgable people on here and some good articles and stickies to read about this. Algae will sometimes come up, especially in newer systems. Even experienced people have to fight it from time to time. Anytime you add some new element, like new lights, a new filter, a school of fish, different ferts, whatever... it seems like algae rears it's ugly head. *sigh* once you find the right balance of light schedule, feet dosing, co2 level and water change regimen it should get much better for you.
