With slow growing plants like swords, and low light, you may be able to get away without fertilizing. Excel adds carbon and is a type of substitute for co2 injection, but does not add what most people are referring to when they say fertilizers. Besides carbon, plants need nitrogen, phosphate, potassium, iron, calcium, magnesium, and other traces. However many of these things are present in tap water and/or are excreted by the fish so I think you can wait and just see how things go. However, if your nitrates are truly bottoming out, you will need to eventually fertilize, or increase fish load.