how large of tank, how heavily planted is it? do the plants look nice and healthy? if so, maybe you arent currently in need of fertilizing. if the plants are looking unhappy, maybe do a water change, and then fertilize. your nitrates might be 50, but that does not mean that the 50 is not lacking specific nutrients the plants need. ill use a 55 gallon tank as an example here. we'll go with 50 for nitrates, and the plants dont look bad, but not looking good either. in this situation, i would probably give 2 squirts of easy green a week, and monitor how things go. directions on the easy green say 1 pump per 10 gallons, but that is really just a decent all around ball park starting point. some tanks need more, and some need less. dont be afraid to partial dose if thats what is really required. hang tight though, there's better people with plants here than me.