Are you from a saltwater or discus breeding background? I ask because it's usually folks coming from reef or discus breeding setups that have this "Nitrates and phophates should be reduced to ZER0000!!!11!0ne!!" mentality.
As the previous posters have said, no need to worry about nitrates -- it's food for the plants. One less nutrient to worry about getting limited and consequently triggering an algae growth.
Also, how are you getting Nitrates to go up 20ppm in 3 days? Unless you're actually dosing nitrates or evaporating and topping off ridiculous amounts of water, I don't see how you can raise nitrates up 20 ppm in 3 days without overfeeding.
In three days or so the nitrates will go from about 20ppm to 40ppm. I do not overfeed. and have only about 25 small fish different varities. I normally do a water change every 5 to 7 days to try to keep the nitrates down to a minimum which In my case is about 20ppm.
While I'm not endorsing this, but I have recently tested nitrates on two of my neglected tanks -- a 10g RCS tank, and a 20gal long that had glo-light tetras and a few endlers. The 10g RCS tank, based from the API test kit, had somewhere between 40-80ppm of nitrates. The 10g, while having lots of java moss, was only getting spill over light from the other tank. The 20gal long had 80ppm nitrates, where the 4 year old glo-lights are showing their age, but still quick enough to eat most of the endler fry to control the population. So, I say don't worry about nitrates if you can keep a huge chunk of thriving plants.