i always have osmocote tabs under the plants.
i dose Kent fert like i said which has nitrogen source/nitrates and all the other useful stuff like magnesium, boron, sulfur.
my tank water has no toxic forms of chlorine. i always treat water with prime before putting it into the tank. chlorine is converted to a safer form for fish.
also i am quite high on bioload with fish (mostly plecos) and tons and tons of snails that poop everywhere. so there are lots of phosphates, nitrates, and carbon sources available for the swords. i do not get why they'd be curing. older leaves are not deteriorating yet.