Plants and NitrAtes

At what point do plants start to consume nitrates? I asks because I had a bit of sadness today. While I have been dosing daily with Brightwell Aquatics FlourinMulti and FlourinAxis at 6 drops per day each I had been holding off of water changes. Last night I had noticed my favorite angel hiding in pants. Not abnormal but she barely came up to eat. She always comes to eat. Since I had been planning on doing a WC today I thought nothing of it. I found her sucked onto the filter intake. I quickly turned the filter off and she floated to the top. She had found herself resting in one of the crypts and I had tested the water. I came up with 0 nitrIte, .5 ammonia (tested 2 more times and both subsequent tests were 0) and 60 nitrAte. I did about 45%-50% water change. It had been 3 weeks and 1 day since the last. I know 60ppm of nitrAte is not low but it is not exactly deadly either. The reason I had extended the time between water changes is because I had been having difficulty with my plants growing and I was trying to keep as many nutrients in the tank as possible. I had since remedied that situation. I know that this could be an isolated incident as all other inhabitants seem normal. My understanding was that the plants would use the nitrAtes in the water thus lowering the concentration. Is this not true? I am very sad because my angel died and she was my favorite. I just don't want to make a mistake again. Please don't be harsh at my 3 week wait between WCs. I usually don't go more than 7-10 days.